I think a very loud minority say that; from what I can tell, most players enjoyed both.
I know I did. ME2 [censored] up some things that were good about ME1, and greatly improved on things that svcked in ME1, but I very much enjoyed both games anyway. I haven't heard many good things about ME3 yet, but that is because I deliberately try to learn as little about it as possible, as to not spoil the surprise. But some of the whining is so vocal that I got wind of it regardless. Still, I feel quite confident that I will enjoy ME3 as well. I just hope that they make the Adept class as powerful as in ME1 again, I hated being forced to use weapons in ME2.
As for trading in Skyrim, I would personally never trade in any game I've bought and I never understood the people that do. The amount of money (or discount on your next purchase) that you get is close to negligible. Not worth it, considering that I always, at some point in the future, feel the desire to replay old games, even when I was sick and tired of them before. But then again, I never considered Skyrim worth buying in the first place, so I wouldn't feel bad about getting rid of that I suppose.