For me it was simply the fact that it's an MMO. Nothing will ever compare to your first MMO - if you first played EQ, then nothing compares to EQ, if it was wow, nothing will compare, etc. For me it was vanilla wow. They've improved the game in almost every way but it's not the same experience.
I never got that immersive feeling from swtor, something wow was great at. I didn't feel like I was in a galaxy that I could explore, I was in a series of levels(planets) separated by loading screens.
Other than having voice over dialog swtor added nothing to the genre.
Everquest was my first MMO and I still prefer WoW over EQ. The camping for spawns got really old in EQ and alternate advancement to stay in high end raiding sucked . Vanilla WoW and BC were better than EQ. I quit and came back a couple times in Wrath and Cata, but it wasn't the same experience for me.
There was no reward for exploring in SWTOR open zones of nothingness. Open World PvP what a joke. Nothing beats Vanilla WoW for that experience. Ilum was a disaster through its 4 iterations.
Problem with MMOs and what keeps playing playing is always being able to advance your character. You cleared your raid this week already and it is Tuesday? You already have top tier PvP gear? All your left with is PvPing more for zero benefit or farming money. MMO's are essentially chase the dangling carrot for better gear and when you are slow at keeping this chase available, inevitably the game will fail.
I won't even get into the HORRIBLE, HORRIBLE performance of SWTOR. You can run Crysis on max settings NP? Well, guess what your going to lag in SWTOR!