The open-world sandbox type games are my favourite. I'm sick of games ending after ten hours of gameplay and how they tend to force me to take a particular course of action. I like the freedom and re-play value that sandbox games provide, and I'm sure I will be playing Skyrim for quite a while.
I played Fallout 3 and Fallout New Vegas for over 250 hours each and I'm sure they will both get re-played in the future. I didn't play Oblivion for that long but I still got a good 70 hours out of it. I'm sorry to say I haven't played Morrowind.
I think I heard somewhere that when Skyrim comes out the bulk of the Bethesda team will start working on Fallout 4, which is pretty exciting. But knowing how long these things take to develop, I'm not holding my breath for a release date before 2013.