Doff system and just about everything it entails. Alternate progression, unique rewards, addictive mechanics, and build complexity through the active roster*
The new skill system has its rough points and could use some changes, but the simplification is conceptually better than the old one*
PTW claims aside, the new consoles add some very welcome power diversity and tangible build complexity at endgame.*
New STFs are enjoyable, repeatable, accessible. Generally a plus, but their storyline is disjointed and unclear now. I think adding another front after the Undine front is in order, one-time missions with the objective of playing X STF for a one time extra reward (maybe a rare salvage or several common salvage).
*- those three might sound contradictory, but my point is that complexity in the passive skill points don't make for interesting or compelling complexity. The active roster actually changes the function of many powers, not just upping their damage or duration, and the consoles add completely new things, moving a lot of build complexity out of passives that make you better and into things that actually make you different.