Tobold asked this question a few days ago and I had meant to reply but was busy with PVP over the weekend. First and foremost I'm a player. Not in the sense of the title to this post, but in the sense of how I interact with MMOs. I'm not one to "shop around" and play test every game that is released because unlike Tobold I do like to accomplish things. He doesn't believe you can actually accomplish anything in a game but I disagree. Accomplishing specific feats in an MMO is the same as completing, or "winning" an RPG or RTS in my view.
So in the sense that Tobold asks the question, I'm the "married" type. I find what I like and I stick with it. I'm the long term, immersive player.