The whole point was to do something different, giving each character their own movie as the background was ingenious. It beats adding 30-60 minutes to the movie to try and fit it all in. They had a plan from the start and they followed it. Not to mention it's been known since the first Iron Man, so you have no excuse to not be up on this.
And how is this not the first mission? The familiarity between the characters is because of the after credit tie ins in each movie and the fact that it all happens in the same universe! How many super hero movies openly acknowledge the other heroes residing in the same universe?
And the Loki questions are answered in Thor
there's nothing "ingenious" about a movie that can't stand on its own. then telling noobs to watch 10+ hours of other movies just to understand it (especially when the story is barely coherent and requires guesswork, even if you've seen the others). that's the easiest thing you could possibly do. the way godfather 2 handled the past, for example, is "ingenious." avengers is basically just an excuse to have a bunch of superheroes on screen.
and during the movie, fury says that he wants to bring the avengers together, but a govt official - who sounds pretty disgusted about what he saw the first time around - reminds him that the 'avengers initiative' has been canceled. maybe 'mission' was the wrong word, but there's obviously some team history that the audience got left out of. i'm not talking about [some of] them simply knowing who each other are.
the loki questions are answered in thor? where? last i checked, this dude was a butthurt adopted kid stranded in space/invisible. then i watch avengers and he's not only present, but randomly pissed at earth people and on some kind of political kick
article about the movie's holes: