Personally, I didn't like either of the Fantastic Fours. That's an example of a film IMO that got a sequel because it made money, not because the first movie was good enough to need a sequel. It probably didn't help that I was never a big fan of the comic, although I must admit I didn't really that many comics outside of X-Men when I was a kid.
Obviously, I'm not naive enough to think that even a good movie will get a sequel just because it's a good movie. Sequels are made because the studios believe they are worth the investment, and thus they only think that when the first film makes a certain amount of money.
There was a long time where comic books weren't lucrative to studios. Other than the Superman movies and the early 90s Batman movies, not a lot of material that was successful that was translated from comics. Then Blade (1998), X-Men (2000), and Spider-Man (2002) came out and were successful. And then Marvel got the idea that they were going to make all of their comics into movies. And because of the success of those movies, studios ate up those ideas and starting churning out Marvel-based movies ever since to mixed results:http://en.wikipedia.org/wiki/List_of_fi ... vel_Comics
And I'm betting that going forward we're going to see more movies like Daredevil, Ghost Rider, and Fantastic Four, which IMHO are mediocre at best films, than we are movies like Iron Man 2, Spider-Man 2, X2, etc., which I'd probably argue are the best of the films produced so far. And notice those are all sequels, but IMO they are well-made sequels. But I think they are more about making money than they are pickign and choosing materials that can make really good movies.
I'm excited about the Avengers because it's unique in the sense that it will take the main characters from several major films and put them in one movie. But I'm not convinced that it will be a good movie, unless Thor and Captain America can produce likeable well-made movies so that we go into the Avengers with the 4 main characters: Iron Man, Hulk, Captain America, and Thor are all likeable characters that we can root for and overcome whatever obstacle is thrown at them. We'll have to see with Captain America and Thor, because you wonder if they are movies that are being made just so they can make Avengers, or are they really good movies that can stand on their own legs. If it's the latter, then great. If it's the former, then we're all screwed.