If it is basically the same, but it improves on things the previous game left unexplored, it is a good thing. For example, one of the problems of God of War 1 was the relative lack of variety in terms of stages and bosses, so the second game is pretty much the same game, with what feels like a lot more content.
If the changes doesn't add too much or the additions are not significant to the core experience, they dilute it. That is one of the main problems I had with AC3... Too many systems, many of which added nothing and were barely used (do you remember separating people during fights? or spurring pigs?) make the game felt like the playground of too many designers.
On the other hand, it is a personal thing. Annual iterations tend to exacerbate the problem more, so seeing it in games like FIFA or COD is far more common; but I have seen people complaining about this with Fallout 3 and Oblivion, even when the games are relatively different and several years apart.