I am a huge comic book collector, so I figure I'll throw in my input. The first Hulk movie, directed by Ang Lee IMO was a very good movie on its own, but not a very good "comic book" movie if you get my meaning. Marvel distanced itself from that movie and had an actor change in the latest Hulk (2008) movie to be a part of the Avengers franchise of films. The 2008 Hulk wasn't very good either (the worst of the new Marvel-produced movies by far), so they had a third actor play Banner for the Avengers films. From IMDB "According to Kevin Feigie, due to positive reactions to Mark Ruffalo's Banner in The Avengers, a sequel will be made after Avengers 2." They have plans for Thor movies, Captain America movies, Iron Man movies, and Avengers movies until the end of time. Nick Fury, Black Widow, The Hulk, and Hawkeye, don't have any solo movies in the works AFAIK.
The X-men, Wolverine, Deadpool, Blade, Daredevil, Ghost Rider, The Punisher and Spider-man films feature Marvel characters, but they aren't produced by Marvel Entertainment so those characters won't be featured in any Avengers movies until those licensing deals expire.
Another quote from IMDB will likely explain it better than I did: