Most of the best-selling games these days are sequels. A lot of people don't like that.
I think it's a sign of the industry's health: it's a good sign that the first game did well enough to fund the second game, the third game, however many games come out. It's true that it seems like most people only buy sequels to games they've already played, but that doesn't mean there are no new, creative games. It just means they aren't as well marketed. In many cases (Psychonauts, cough) this dooms them to die a horrible death, but that's true even if there are no sequels running around.
For me, I like sequels because I know what to expect. I'm a little disappointed by how WELL I know what to expect, but that's a different matter. The point is that, in general, sequels are strong games made by a strong, experienced team working with an IP and tech base they are very experienced with. They're successful because they're usually quite good.
If I want to be surprised by something, I buy something that doesn't have a number in the title. Usually, I buy strictly indie games on that front: I'm nervous buying anything like, say, Mirror's Edge, because they tend to have the worst of both worlds: an inexperienced team working beneath a "creative board" that cripples any creative impulses. Creative games with inexperienced teams are fine, clones with experienced teams are fine, but clones with inexperienced teams? No thanks.
(Obviously, Mirror's edge wasn't a clone... but it was definitely rough, especially in the "writing" department. I put it in quotes because I would hesitate to call that "writing".)
For me, there is an interesting edge to new games, games that aren't cycling through an old IP. And this relates to my last post about "full characters", actually. A new game will typically revolve around some powerful organizing concept (often the main character's weird abilities) and will therefore have a very unique flavor.
Even if an IP is quite good, that doesn't happen in sequels. The powerful organizing concept might be there in the first game, but after that, it's pretty familiar, pretty well explored. There's a push to keep the nth game feeling like the nth-1 game, and that means that the edge wears off even as the team starts to come together and polish the game to a shine.
I wonder if it's possible to build an IP that explores weird new games, an IP that keeps its edge no matter how many games you release. I have a sneaking suspicion that even if you managed to come up with a way to do it, it would be instantly derailed the moment you became a success as the well-meaning (greedy) board of directors gets its talons into the project.