I think it will most likely suck. Joss Whedon is the only thing that gives me any hope that the movie might be okay. And his track record is a bit spotty. He created Buffy and Firefly, but he also created Dollhouse. He directed Serenity, but that's the only feature film under his belt. It's not much to go off of. There are plenty of directors out there who have one great film swimming in a see of mediocrity on their resumes. Moreover, Serenity was a small, personal, fairly inexpensive film based on his own creation. The Avengers is a $150 million tent pole film which means that Whedon is going to have producers following his every move, saying things like , "Oh . . . you can't do that," and, "Hey, we really need you to have a love-triangle in the film so this will go over well as a date movie," etc. He's going to have back seat drivers on this film that he didn't have on Serenity.
On top of all that, his work is cut out for him. When a comic book movie tries to follow too large a cast of characters, it almost always ends up giving a shallow treatment to each (because there's just not enough time) and the movie is bad. Hell, Thor had super shallow character development when he had an hour and a half all to himself. The first two X-men movies showed that it can be done. But it usually is not done well. Even Sam Raimi, who proved that he can make an excellent comic book movie (Spidey 1 and 1) faltered when he tried to cram in more characters (Spidey 3).