I've never played an MMO so Fable 2 was as close as I've ever gotten. Running through such a huge world and seeing other people online, was one the most awesome experiences I've had this gen. Interacting with them, getting/recieving gifts, talking with them about puzzle solutions, and other things. It was great! I'm hoping that Fable 3 improves upon this.
I think F2 was worth owning a 360 for. If F3 is better, then you won't regret your purchase. See ya online!