That when making a series, they keep what's good and throw out what's bad. For example, the Crash Bandicoot series. For the second one, they took what people liked about it, but toned down the difficulty, what people seemed to be against, then added new things which offered a better and different experience, but not too drastically different. And for the third one, they took what made Crash 2 fun and successful, kept them all for Crash 3 and added things which offered a different experience, but didn't change it that much it made it into like a different game. Let's compare that with Halo. Halo 1, was a large success. Halo 2, they took Halo 1, and seemed to change a hell of a lot in it. Then for Halo 3 they took it, changed some of the mechanics and added equipment. Then they just went derp for Reach.