The Dev Blogs

Dev Blog 1 - Announcing Our Biggest Game Systems Update Ever!

The first dev blog that announces the new "Talent" system.

Every hero is getting a Quality of Life update as part of the new systems.
Powers no longer use Power Points and will have the former benefits of maximum Power Points incorporated directly into them automatically.
Items will no longer give Power Ranks. What will replace this is still to be announced in the next Dev Blog.
All heroes will now be restricted to a single 8-slot Power Bar for slotting active powers.
The old Crowd Control cleanse powers have been removed and incorporated into "Movement" powers like dashes and teleports.
Talents. Heroes now have 5 sets of talents that unlock as they level. Each set lets the player pick one of three choices that will alter powers or give other bonuses.
Traits. Heroes will now have an offensive and defensive "Trait" gained at 1st level. Through their Offensive Trait, all heroes have two Attributes selected that give them a scaling damage bonus based on how much of that Attribute they have. Defensive Traits will contain the Total Defense/Deflect/Dodge multipliers and +8% Damage Reduction affix currently found on defensive passives. Both will also contain a "flavorful" affix to make the hero more unique. Some heroes will have a third "Mechanic Trait" that explains their secondary resource or other complex mechanics unique to them.
Attributes are being overhauled "with the goal of making all Attributes useful for all Heroes." More details in a future Dev Blog.
Contains examples of these changes, including new powers for Luke Cage, Loki and Human Torch.
[Official] Hero Power Update FAQ. Check here first for truly official answers. @AgentVapor went and posted it while I was still working on mine. Totally ganked.

Dev Blog 2 - Items

[Official] FAQ Thread

Power Ranks

Items will no longer have bonuses to power ranks, as power ranks no longer exist.
"All Items and recipes that grant Power Points are being reviewed and updated."
"This also includes the cosmic tier of Cybernetics, which will get some fun new powers."
Most power rank affixes will be replaced with bonuses to Attributes.

Uniques

[Official] Item Update - Info on Unique Changes for Crafting, Boxes, Drops, Achievements, etc
"Unique slots one through five Items will no longer drop in game."
You will not lose your existing uniques. All existing uniques will be reviewed "to ensure their continued viability".
Instead of uniques, the top tier of slots 1 - 5 will be Omega Rarity items.

Omega Rarity Items

Omega items replace Uniques as the top-tier slot 1-5 items in the loot table.
They only drop at level 60.
All Omega items are character specific. There are no "Any Hero" Omega items.
Similar to white/blue/purple/cosmic items in slots 1 - 5, Omega items have random affixes, unlike Uniques.
Random affixes on all items (both Omega and lower rarities) will now be filtered so characters cannot get completely useless ones, such as Summoned Ally Damage on a character that cannot summon.
All Omega items contain an Omega Power. Omega powers add new effects or alter the mechanics on one of your powers.
Examples were given of Omega items, but so far these are mockup placeholders and not representative of the finished item power levels.

Dev Blog 3 - Infinity System

The Omega system is being replaced by the "Infinity" system.
Rather than 14 pages of options there are now only 6, each represented by one of the six Infinity Gems.
Within each "gem" are 5 options in which to invest your Infinity points.
One option under each gem is an "infinite" option, which has no maximum. You may invest as many points as you like in them.
Existing Omega points will be converted to Infinity points at a rate of 10 Omega points to 1 Infinity point (rounded up), to account for Infinity nodes costing significantly less and being more powerful than Omega nodes.

Dev Blog 4 - Mobility

"Dash" powers (including teleports, rolls, and similar quick "click to move" powers) will now all have fixed distances and travel speeds.
"Dash" powers will have three charges, with a four second cooldown per charge as a baseline. Some heroes will have more charges, shorter cooldowns and/or ways to regain charges in combat.
Dash powers now grant invulnerability and remove crowd control effects while traveling.
Teleports will now cover the same distance as a dash and do so just as quickly.
Channeled travel powers will be reverted back to a speed of 700 and must now be channeled for 1s before they start.
"Ziggurat of Kargul will have a new ability replace its active power."

Dev Responses, Clarifications and FAQs

Keep Calm

Prestige and Omega progress will not be lost. [source]
Ultimate Powers will not be changing, will continue to have 20 Ranks and Signatures will continue to receive bonuses from Ultimate ranks. [source 1, 2] Items that add Ultimate ranks will still do so. [source]
Your hard-earned items will not be lost. [source]

Powers, Power Trees, Talents and Traits

Talent choices are mutually exclusive -- you may only pick one from each of the five sets -- and can affect categories or sets of powers. [source] (Example of specializations converted to Talents using Cable)
Talents replace all former passives, toggles and specializations. [source]
All existing powers have been "slid over" to fit into two power trees instead of three, freeing the third panel for Talents. Very few if any powers have been removed outright. [source]
@Doomsaw says the new changes look "awesome". [source]

Attributes

"The attribute system is changing so that while the attribute still gives specific stats, you will almost always want to gear for your hero's primary attribute, which may not be Intelligence for summoners." [source]

Items

General

No item rarities will be removed from the loot table except "Uniques". When those rarities drop are will be defined by a "a pretty smooth curve of item power increases as you level, instead of getting cosmics or uniques at level 15 that makes you ignore item drops as you level." [source]
About 70% of the existing random affixes on slot 1-5 items were power points or power grants, and will be replaced with entirely new affixes. [source]
Power Ranks will be replaced with new properties, mostly attribute bonuses. [source] This includes Cosmic items and their +1 ranks. [source]
Loot tables are receiving a massive overhaul. Pet Cybernetic requirements will be lowered to match the move towards dropping "less undesirable loot". [source]
"We have work being done to make RIF better, including allowing it to impact boss specific chase item drop rates that are either coming with this system update, or shortly after. We want to make RIF and SIF great again!" [source]
Move Speed will not be removed from existing items (to prevent rerolling), but future Slot 1 - 5 items will no longer receive it as an affix. [source] Other slots may still get it and it will be available through the Infinity System. [source]
No plans for "set items". [source]
"We aren't anticipating the changes to items to cause massive rerolls on items. Power points(the main things removed) are controlled from different item seed than other affixes. I cant promise that none of your items will be re-rolled, but I am very confident that it wont be a problem. However, new attributes being added to items will not inherit old power point rolls and will be given their own new percentages." [source]
Champion commendations will one day totally be useful. [source]
"We are looking in to Power Duration and agree with your assessment that the amount available can break a lot of things. When we get closer to TC, we will have more information on our plans." [source]

Medkits

The Medkit button and affixes will continue to work identical to how they work now, but will no longer require Medkits in your inventory to work. [source]
Medkit related achievements are moving to the "Feats" section with other defunct achievements. Do them now or never. [source]

Artifacts

Items that can be slotted to the Power Bar and activated will be losing this functionality.
"We have designs to make all of the 'level up' artifacts much more viable while leveling and fit builds way better. I'm not sure if that will make it in with this system update or not, but it should within a few months or so." [source]

Uniques

Important: The Unique to Credit Chest exchange recipes will be removed during the Talent Update, not the later update containing the other item changes, in preparation for an event that will help players finish up unique-related achievements. [source]
"No uniques are being removed. Actually, Luke Cage's Disco Shirt is actually becoming a real unique :o" [Sweet Christmas!]
Uniques with high power rank rolls will not necessarily get good Attribute rolls. "Power point affixes and attribute affixes are handled totally differently." [source]

Omega Items / Item Rarities

They will have 5 random affixes. [source]
Which affixes you get on Slot 1 - 5 items will be random, but the values on those affixes will always be the same. Higher values will be achieved through finding higher level items rather than rerolling. [source]
Values on affixes will be the same across all rarities of the same item level (ie, if an Epic level 63 item gives 2,000 Max Health, an Omega level 63 item will also give 2,000 Max Health). Higher rarities will simply roll with more affixes in total. [source].
How often Omega items drop is still to be determined, but will be affected by the Difficulty Slider. [source]
Higher level Omega items will drop at higher difficulties. [source]
It will be possible to upgrade low level Omega items to higher item levels, but will require crafting materials that drop at higher difficulties. [source]
~~Omega Powers are tied to specific items. For example, every Captain America's Zero Point Energy Shield drop will increase the number of bounces on Shield Bounce by 3. [source]~~ Design changed. Omega Powers will likely be a pool of random affixes that can appear on multiple slots. [source]
Omega items, like other Slot 1 - 5 items, will be account bound. [source]
Characters will launch with 5 to 7 possible Omega powers, but many more will be added over time. [source]
Additional "generic" Omega Powers will be available, which can appear for multiple different heroes. [source]

Legendaries

Possibilities for new Legendaries. [source]
Norn Stones will be updated. [source]
Other existing legendaries will not change in this update. [source]
Movement-based Legendary? ";)" [source]
Rare boss specific Legendaries are being worked on. [source]

Starktech Cubes

Starktech Cubes will be going away and existing ones will become Ancient Forgotten Devices to be traded to Clea. A few new things may be added to the vendor list. [source]
The new items will not be announced ahead of time. [source]
Engineering isn't complete, but the goal is to count both slotted and unslotted cubes. [source]
The previous hinted update to them has been scrapped for now. [source]
There are no plans to bring the cubes back. [source]
Exchange rate is still to be determined. [source]

Miscellaneous

The fate of Story Mode Health and Spirit bonuses is still to be determined. [source]
"We will be removing flat size increase or decrease from items. Our modelers and vfx artists have begged for this for a long time, as it makes building visuals, costumes even more time consuming to have to account for different sizes. We plan to, for the most part, move these instead to procs on the item to grow/shrink for a few seconds instead of being huge/tiny all the time. We know some players really enjoy these affixes and sympathize, but the back end downside is too great." [source]

Stuff Coming Later (Not This Update)

The free blessing swap will be extending "a month or two post this update". A cost will be added later. [source]
Changes to the way costumes are handled/slotted is in the works, but not until after the current system updates. [source]
"No changes to costumes/cores/affixes in this update. But there will be some in an update soon after. Will discuss more as we get closer to that." [source]
Relic changes are not a part of this update, but likely afterwards. [source]
The ability to reroll single affixes instead of an entire item is being worked on, but will not release with this update. "It also won't be cheap to do." [source]
There are plans for a system to differentiate between costume core rarities in development. [source]
"We have some plans to simplify the rune system and clarify a lot of it. It wont happen for the system update, but shortly after, if all things go according to plan." [source]

Other Stuff Called Out As Not Changing This Update

No new or altered Runewords beyond Attribute and Power Point changes, but the itemization updates will open up opportunities for more later. [source]
Crit bonuses "will receive a balance pass of sorts, but will remain mainly unchanged." [source]
No changes to the Summoned Ally Damage affix. [source]
No changes to Team-Ups. [source]
No changes to Eternity Splinters. [source]
Prestiges higher than Cosmic are being considered, but not until after the Talent launch. [source]

Heroes

I didn't have the foresight to reserve extra space. Moving the heroes to a new thread.