Story, characters, atmosphere, dialogue. Some of the dialogue can be pretty funny. Story is pretty good as well, not perfect.
And compared to other mmo I really like the combat. The abilities feel like they have impact and are powerful, good animations. I really like using the channeled stone shuriken on my ninja (sage), and it's pretty fun to scream UNLIMITED POWER at my screen when playing my sorc. Armour can look pretty ****** as well. I just seem to connect more with the armour styles in swtor than in most mmos.
The art style in the game is pretty good as well. It ages really well, and looks very smooth.