Imo the only good thing about the last patch (other than the new look of Avengers Tower) was the XP boost on story mode. Some people have been complaining about XP being excessive, and that led us to an incoming XP nerf. I don't think story mode XP should be nerfed, and here is why (not that I think my opinion will change anything, since nerfs have become a tradition):
I have all characters in lvl 60, what leads me to a 200% XP boost - 210% with Cyclops synergy bonus. To level from 1-60, I gotta kill all story mode bosses till Malekith (+50% xp boost from Taskmaster reward potion). Makes me think a new character will have to complete all story mode to reach lvl 60. Sounds about right to me.
People have been comparing this to other ARPG, saying it is too easy to reach lvl 60. Well, this is true, but I have never played an ARPG that has nearly 60 playable characters. Also, for people who like leveling (myself included), to "fully" level a hero you gotta go from 1-60 "31" times (white, green, blue, purple, orange, red, and yellow, that needs 25x the normal XP). So if you are a player that doesn't care about prestige, the amount of XP currently provided might seem excessive, but for the players who like prestiging, it still takes a bunch of time until you are cosmic 60. Yeah, that's not mandatory, but lots of people like prestiging.
Reaching lvl 60 is not the end of the game. As in most games, it is just the beggining. After reaching 60, you will farm uniques, gather artifacs, relics, legendary (and level it), uru, insignia, armor core, ring, medallion and feed a pet. There is a bunch of endgame content for that: cosmic challenges, danger room, raids, terminals and so on.
It would be an excessive ammount of XP if in your first hero you could go from 1-60 in under one hour. But this is not the reality, unless you are being carried by a higher level player. People reaching 60 in under one hour have a huge 200-210% boost that was acquired by leveling a bunch of other heroes. People who claim to reach 60 in ~20min are running a ton of boosts. And leveling faster is exactly the reason why they spend $$ on boosts.
People who want to level slower have the option to disable synergy boosts, removing the 200% XP boost provided.
For people who only play their favorite/available heroes, the time for leveling might be too short. But for people like myself who intend to eventually prestige all heroes to cosmic, the current XP seems about right. I used to level/prestige only when serverside XP boost was available, so I could make my expensive as hell XP potions worth the money. The change made me feel like prestiging all my heroes to red doing story mode so I could start prestiging to cosmic one by one (I currently have only 11 heroes on cosmic prestige). The cosmic prestige won't be too fast, since doing all story mode would still not be enough to reach lvl 60. That means you can quickly go to cosmic and then slowly level the hero, even considering cash store boosts. I estimate it would still take around 8-9 hours to go from 1-60 in cosmic.
Concerning having what to grind, leveling is the least important concern. You will spend tons of time grinding gear, currencies, money/molecules to reroll stuff and so on.
Since prestiging and achievements have been implemented, I usually play new heroes to white 60 and then chose other heroes to prestige till red. During cosmic events, I chose a red hero to cosmic prestige to enjoy the faster leveling. After I'm done with that, I start doing every single story mode mission and all hero achievements for that hero. I also try to gather high end gear (BiS artifacts, best insignia I can get, lvl 80 legendary and so on). So there is a LOT to do after you reach lvl 60, even if it is cosmic 60.
With the current ammount of story mode XP, I was compelled to play more - what also means spend more money. With the upcoming nerf, I will probably go back to "play once in a while" mode, since non-cosmic challenges are pretty unbalanced at the moment and I need vacations on legendary quests.
I can't see a proper reason for the nerf other than ruining the fun of people who are enjoying this, since the ones that are not liking have the option to turn off synergy bonus.