I've sifted through many angry comments about the limitations placed on the Avengers roster and this is my reasoning: it's for the better. Pure and simple.
Ant man and Wasp simply don't fit into the movie. I mean, which would you rather: a movie full of their development with a lame villain haphazardly thrown in, or a well developed plot that mixes the growing relations between already established heroes with the development of an interesting and credible opponent
You have to choose. You can't have both. This a movie, not a comic. There simply isn't time to develop fully two new heroes AND a villain in under three hours. Either Giant Man and Wasp would be under-developed and misused, or the villain would make no sense.
Which would you rather: a mediocre movie with all your favorite heroes, but with some of them misused and done poorly; or a great movie only without a few minor heroes?
I think the choice is clear. I mean outside of the ultimate universe, name one instance where Giant Man and Wasp REALLY contributed anything astonishing? They're flat characters. If they were in the movie they would be like their ultimate counterparts.
It was pointed out that without them we wouldn't have Ultron, who is decidedly one of the Avengers most important enemies. However, this cannot be achieved in a single movie, and certainly not in the first Avengers movie. Do you want to see superfluous scenes where Giant Man is working on his artificial intelligence program? Sure it'd be an interesting tidbit, but would it forward the Avengers storyline? No. Such a thing would be better off in a Giant Man/Ant Man movie. It would work there because the movie would be focus on him.
The Avengers is a team movie. As such, it has to be about the team, not so much the individual members. It's sad that the Wasp and Ant Man won't be part of the first foray. They are not established characters (to the general public.) You would have to devote a significant amount of time to them, time the director doesn't have. A 30 second intro isn't enough for these characters, as they are not as well known as the zhulk or the X-Men. Those short intros work because everyone knows them already. If I asked a room full of comic outsiders to identify the characters in a picture of the Avenger's: do you think any would name Giant Man and Wasp? Furthermore if they could, would they know anything about who they are and how they got their powers? The answer: no.
If you still think a 30 second intro of an unknown character can work, I point to exhibit A: Wolverine origins. How did everyone enjoy the non existent character development in that movie? That is an example of how a team movie can go wrong. Do you want the Avengers to suffer the fate of Wolverine, or do you want a well developed plot and interesting character development?
Choose wisely.
I know this is a controversial topic. I look forward to the exchange of ideas.