going by hints from @Asros it sounds like the higher on the slider you go the better loot etc. so whats stopping one from going to high tier on slider, gearing up well, then bringing that slider back down and one shotting everything in lower tier midtowns etc.
the difficulty slider if it works like diablo's torment etc hasnt been done in mmos and i can see alot of griefing happening
unless they are making midtown instanced for everyone