For one, this is an actual problem in the comics that appears to be stretching over into the movies, where the Avengers is not a team of super-powered world-savers, but instead is just the most popular or the highest-selling characters. I think perhaps the worst example of this, from both the comics and the movies, is the Hulk. The core concept behind the Hulk (again, both the comics and the movies) is that when Bruce Banner gets angry, he turns into an unstoppable, uncontrollable monster. That just does not translate into "team player". At the beginning of The Avengers, Bruce Banner is hiding out in...India or someplace, because he's afraid of getting angry and hurting people. This is what the Hulk is all about. But then, at the end of the movie, we see Hulk standing around, taking orders from Captain America, and apparently Bruce Banner is able to control when and how he transforms for the sake of the plot ("plot"). Instead of trying to work Banner/Hulk into the Avengers as a team player, just leave him out of it. That's not his place.
(If you absolutely had to have The Hulk in The Avengers, how about bringing him in as an adversary? He is either being mind-controlled by Loki, or Loki brings Banner into the middle of a fight, knowing that he'll take collateral damage somehow, get angry, and fuck everything up. The Avengers can't take on the Hulk individually, so they have to work together as a team to overcome this challenge, which adds to their character development, and the Hulk is still just a rampaging monster, which maintains the character that has been developing for the past decade.)
The Hulk is the worst offender, but sadly not the only one. There was one time when Storm and Wolverine were both on the Avengers, and while I'm sure they care about saving the world, I think their characters have been developed more to the point of taking care of the Xavier/Jean Grey schools, and working on human/mutant relations. Now, Wolverine, Havok, Rogue, Sunfire and Scarlet Witch are part of an Avengers team with Thor, Simon, and the Wasp, working to improve human/mutant relations. This makes a lot of sense to me, especially since none of those mutants are really the "big damn hero" type.
Spider-Man, Daredevil, Luke Cage, Hawkeye and a few other "street level" heroes are also of this nature. In any narrative, the threat to the heroes should be commensurate with their abilities. One of the best Spider-Man stories ever told was about Spider-Man trying to take on the Juggernaut. This was a really interesting story because Juggernaut is just outside of Spider-Man's strength level to make it a great challenge, but if it were the Hulk going in there, or Thor, than there would be no challenge and hence zero interest from the reader. The same works in the other direction too, if the challenge is too big to possibly overcome for the hero, you begin to ask, "Wait, why are you even here?" Again, for examples of this stupid idea put into play, look to The Avengers again. You have Thor blowing everything up with lightning bolts, Hulk is punching through giant monsters, Iron Man is flying around shooting everything with lasers, and then Black Widow is somewhere on the ground kicking people occasionally. In the comics, they had a story arc last year with a huge galactic space war, and wisely, Daredevil did not come along.
But I get it, people are not thinking about these sorts of things when they here that Spider-Man is going to be in an Avengers movie someday. They want to see these characters all together in a movie. I'm going to be totally honest here: I am so confused as to why people think this is something to get excited over. I think it's dumb.
This baffles me so much I have trouble wrapping my mind around it, much less trying to explain it even in a semi articulate manner. Aside from the reasons I just listed, I suppose this reeks of over-indulgence, to the point of being insulting. It's like the studios are shaking me by the shoulders saying, "You like X-Men right? You like Fantastic Four right? What if they were on the screen at the same time? Isn't that amazing!?" No. No it is not amazing. You just took two things that I liked and put them together awkwardly. I promise you that the Batman/Superman movie is going to stink of this principle. "Hey, look it's Wonder Woman! She's on the screen! I have seen her!" This doesn't make for a good movie, it's just images. The moment where The Avengers lost me was when they had Thor and Iron Man fighting for no stinking reason. They just put that scene in there not because it made sense in the context of the plot, but because the filmmakers thought that idiots like me would like to see a fight, which is not necessarily false, but I'd rather something make sense first and be cool second.
Of course, if you feel this to an absurd conclusion, you might eventually get how dumb this is. What if we had a movie with Darth Vader fighting Goku? You love Star Wars and anime right? Let's put them together! With Baywatch too. These are things people like, so let's mash them together and people will like it.
No. We just want a good story.