First, thanks, Seth, for clearing it up that Redemption was based on Red Dead Revolver, which was released back in 2004. I was about to lose my mind when reading it posts saying it was a new IP.
Second, I don't mind sequels as long as the games keep it interesting. I don't hear many people complaining about new Mario, Uncharted, Zelda, Assassin's Creed, and many other games because they are good. In fact, I think anyone could argue that the first AC was utter crap after the first hour due to constant repetition, but since then, has greatly improved their games due to sequels. However, there are series like Final Fantasy that used to be this great, but have since stagnated. I love JRPGs, but FF is even getting rid of some of the innovations they allowed for like non-linearity in order to push a rather mundane story.
Overall, I think sequels aren't bad and can really allow for some great innovation and improvement. If not for the small successes Rockstar had with GTA 1 and 2, they never would have made 3, which was a huge overhaul. Mario has parlayed 2D success into some of the most innovative titles across multiple genres. AC has gone from mundane, repetitive action to a very action packed, beautiful game.