I voted for PvP, Class Story, Ops, mini-games, and space content.
PvP and Ops because they're challenging and tons of fun. Space content because pew pew pew, and minigames because they add immersions and give us a few moments we spend not killing everyone and everything around us.
Now, as for class stories, it's because that's what's good for the MMO. When they released it, it was with the advertisement that this was gonna be the best storylines in any MMO. It was gonna be our "Personal Star Wars Saga". Needless to say, that has attracted a ton of people from the single player crowd, who are only here for the Star Warsy stories.
Maybe if during release they had different goals, this would not be a priority. However they decided to cater to people that like single-player storylines. They have to keep these players, because they're gonna be the most loyal. Class stories are the main unique thing about SWTOR, the one thing that sets it apart from the majority of other MMOs (aside from the SW in the title
). They have to keep releasing new personal storylines to keep that playerbase happy.
Not at the cost of other MMO-features of course, I still think endgame should be a major priority, but Class Stories should be up there as well.