Well, they did have to reintroduce since it's highly possible not everyone saw every single Marvel movie, but at the same time, you pretty much had to have some Marvel knowledge to get this movie. Although they did do some reintroduction, they couldn't possibly tell every single backstory explaining why they are how they are. Since the Hulk is mostly muscle, I can see why Whedon neglected to tell his.
It's not exactly the most noob friendly like Transformers or individual Marvel movies. But it was still an awesome movie that definitely caters heavily to the fans of Marvel. Probably the best Marvel movie in my opinion.
Loki got the job done and the actor did a more than adequate job, since the movie is based on the Avengers overcoming their personal matters to become a team. It wasn't really about Loki.
Josh Whedon definitely knows how to make a comic book movie.