I started WoW for the first time a month ago, and stopped not long after hitting 60. For me, a game's storyline really has to pull me in and make me sympathize with my character or the world I'm in to encourage me to continue playing. WoW did not do this, simply because I did not feel connected to my character at all. I was just leveling him up, quest by quest, hardly noticing the description of each mission before hitting the accept button. I had no incentive to keep playing, other than my next level's new abilities or the prospect of a new mount around the corner. I needed to hit 85 ASAP, and that was my only initiative.
Contrary to this, The Old Republic drew me in from the beginning with its storytelling. The addition of character speech and morality makes my Sith Warrior feel like a fleshed out antihero rather than a mute, generic cartoon, and dialogue choices add an element of personality and customization, making me believe that I really am an evil badass. Gameplay wise, combat is smooth and satisfying. Leveling my character does not seem like a conscious action, it rather flows through the story as my character grows and becomes more powerful. I cannot WAIT to see what the rest of the story holds in store for me.
In short, WoW to me felt like an MMO without the RPG, and I was never a part of that world. No amount of expansion packs, chock full of new dungeons, quests, and abilities could change the fact that I was detached from the world. Now with TOR, I feel like I have become part of the Sith Empire, and I am fully vested in the goings on of the galaxy. I cannot WAIT to see what the rest of the story holds in store for me. If that doesn't keep me playing, I'll go back to Skyrim. But I doubt that I'll stop killing imperial idiots on a whim or torturing innocent civilians anytime soon, and I will continue paying in order to do so.