WoW kept me playing because I was paying for it. GW2 keeps me playing because I’m not.
~B.J.Keeton commenting at Game by Night blog.
I find myself with the same sentiment. When I first started playing MMOs, WoW was it, and I couldn't imagine playing more than one game like that. I was committed both by the subscription itself, and the feeling that I needed to get my money's worth. Over six years later, I find myself reluctant to commit financially and temporally to a game like that. I left off playing TSW because I didn't feel the subscription was worth the time I could commit each month. GW2 is perfect for that reluctance (as is TSW now), I can buy something in the cash shop if I think it's worth the money, but I'm not locked in.