Amongst the large number of fixes, should've beens (from release) and additions, content is king when keeping people.
1 Operation
Only thing that would keep me playing for a few weeks, though no nightmare mode.
1 Flashpoint
Done in a day and never to be seen again
1 Warzone
Good enough, but they removed all rewards from Ilum;
New Dailies, so I can pretend I have something to do everyday for all those credits I don't need to spend.
Also, for existing players and those who filled up every character slot and level a fair bit, the legacy system isn't really going to be worth it that much.
Depending when it's out I might give it a go but my server is dead, there were actually NO guilds recruiting in game or on the forums since the only nightmare raiding guilds quit and mine was one of them. HM guilds was down to 1 who only needed one person.
So it's very likely I'd resub, do the flashpoint and pvp until I
might happen to find a guild then curse at myself for wasting the money.
edit: I just checked, one guild on republic on niman recruiting since I left