It's always a tough balance.. and it really does vary from game to game. Sometimes all you want is a glorified expansion pack, sometimes you want something that takes the successes of the first and completes the package with altered/tweaked/brand new stuff. Personally, I'm always a little disappointed when a sequel to a game I love doesn't really change anything at all. I've also, of course, been disappointed when changes are made and the game loses what I liked so much about it in the first place (the Star Ocean series after SO2 for example). I guess I'll just never win. Hehe
Shamus does bring up a good point though. I'm a fighting game nut, but you only need to look at that genre for a concrete example. I can jump in and play almost any fighter released with a little adjustment period, and I scoff at games that haven't advanced depth wise much beyond SF2. At the same time, however, I realize that part of what made SF2 so popular was the appeal of it being something new that anyone could get into and play, maybe even stealing a win from a vet here and there. Add too much depth and complexity with each iteration and eventually you will make a game untouchable to the newcomer.