Rock, the concept of introducing characters isn't an awkward "we're just showing you who they are again' it's a crucial thing to do in ANY movie, especially a Superhero film.
Think of how they did it in Iron Man, you have the classic case with Iron Man's first entrance, we see Iron Man flying, then we see Stark showing off and then we briefly see him studying his blood toxicity. That scene does so much to further the movie, it reaquaints us with Iron Man and what happened at the end of the first movie, Starks celebrity status and then sets up the future events for him regarding his blood poisoning.
Introducing characters isn't something that detracts from the story or forward momentum of the movie, it's crucial to the movie. It's planting the seeds of all the character development. I think 40 minutes isn't too much either because thats not 40 minutes straight of CHARACTER introduction, but introduction in general, that includes villains, plot layers, setting up potential conflicts.
Also, character development doesn't mean showing each character on their own looking wistful for whatever reason, it can be done any multitude of ways, with other characters present, and Whedon is brilliant at this sort of thing when it comes to limited time and alot of characters and story.