I can no longer spend the amount of time it takes to run 3.5 that it takes me.
I enjoy running D&D, but 3.5 has ground me into dust with DM prep time. I never liked AD&D (1st or 2nd), and my group doesn't like BECMI D&D.
If 4e is quicker prep time than 3.5, and is fun to play, we'll play...