Marvel Studios has done a pretty fantastic job of putting together a film franchise that they call the Marvel Cinematic Universe. When I first heard about The Avengers, I wasn't very excited about it, mainly because I never read any of the Avengers comics in the first place; also, I assumed it would just turn out to be a jumbled Schumacheresque mess of a movie that did none of the characters any justice anyway. Boy, was I wrong.
Thankfully, Joss Whedon turned out to be the one who directed The Avengers, and it ended up being one of the best superhero movies ever made. Even people who don't normally enjoy superhero movies will find lots of things to love about this movie. ( By the way, if you're boycotting everything Joss Whedon does because he said something stupid on Twitter, please do yourself a favor and get over it. Dollhouse is probably the only thing I've ever seen him inject politics into, and even that was still watchable. )
You can still enjoy The Avengers for what it is without necessarily seeing all of the other films in Phase One of the Marvel Cinematic Universe, but you will probably enjoy it much more if you at least watch Iron Man and Thor first--both are solid films with terrific casts that have directors who actually give a shit about what they're doing. I can't think of anyone better to play Tony Stark than Robert Downey Jr., and I've seen Jon Favreau direct good movies before, so I wasn't really surprised by how much I enjoyed Iron Man. What did surprise me was how much I ended up liking Thor; Kenneth Branagh did a superb job of blending what is basically a fish-out-of-water romantic comedy with just the right amount of sci-fi-ish Shakespearean tragedy. ( Some have tried to accuse Branagh of political correctness for casting Idris Elba as Heimdall, but Elba somehow fits in the role so perfectly that it actually makes it hard for me to imagine anyone else as Heimdall. All of the casting was spot-on, really, even the small parts. Tom Hiddleston was especially great as Loki, which is probably why they decided to bring him back again later as a villain in The Avengers. )
The Incredible Hulk is definitely one of the films you can do without. I'll go even further and say that you should actively avoid it, even if you're a completist. As I mentioned before in another post, Louis Leterrier made such a mess of this movie that it makes Ang Lee's deeply flawed Hulk seem like a cinematic masterpiece by comparison. I still don't understand why they made this awful remake instead of just making a better sequel to Ang Lee's movie that ties things into The Avengers. And Edward Norton just ends up being replaced by Mark Ruffalo for The Avengers anyway, which only makes the whole exercise seem that much more pointless.
Iron Man 2 isn't really necessary in order to understand The Avengers, but it's still worth seeing if you enjoyed the first Iron Man movie. A lot of what worked in the first film is still there in the second, but it still felt to me like something was missing. ( Jeff Bridges, maybe? I dunno. ) Mickey Rourke seemed either miscast or misused, and I got tired of seeing his weird face and stupid hair after a while. My favorite part of the movie was watching Sam Rockwell chew the scenery as a minor villain who doesn't know that he's a minor villain, but your miles may vary. Oh, we also get treated to a preview of Scarlett Johansson in her Avengers catsuit, so there's that.
Captain America: The First Avenger was a decent movie, but most of it was pretty forgettable. I have nothing against Captain America, but I liked him as a character much better in The Avengers than I did in his own origin movie. Well, there was one scene at the end of Captain America: The First Avenger that I really liked, but that scene was only added to give Samuel L. Jackson a place to make a cameo appearance. If the best part of your movie is just a setup for another movie, you've done something very wrong. Too many characters felt like throwaways. Even the villain didn't feel like a fully formed character to me. How can you not make the Red Skull, a super-evil Nazi with a giant red skull for a face, more interesting than this? ( And why did he have to spend so much time looking like Hugo Weaving? I like Hugo Weaving, but come on. We already knew from the ads what he was going to look like, so why hide it? )
Just as Phase One ended with The Avengers, Phase Two will end with Avengers: Age of Ultron, also directed by Joss Whedon. ( I assume that Phase Three will lead up to a third Avengers movie, hopefully also directed by Whedon, but I can't say for sure just yet. ) The only Phase Two movies that I have seen so far are Iron Man 3 and Thor: The Dark World, both of which are worth seeing, but without knowing more about the second Avengers movie, I can't really say whether they are necessary or not. If I had to rank the Iron Man movies, I would place the third somewhere above the second but still below the first. I'm convinced that The Dark World would have been a much better and much more coherent movie if only Kenneth Branagh had been kept on to direct, but if you enjoyed the first Thor movie you can still enjoy the second for what it is. ( I haven't seen Guardians of the Galaxy or Captain America: The Winter Soldier yet. I've heard nothing but good things about them, so I'll be sure to catch them both on DVD when I get a chance. )
Amazingly, Clark Gregg has appeared as Agent Coulson in Iron Man, Iron Man 2, Thor, and The Avengers, and now he even has his own show based around the character despite Joss Whedon's attempt to kill him off. Marvel's Agents of S.H.I.E.L.D. was pretty uneven for most of its first season, but it had a surprisingly strong finish thanks in part to a terrific guest appearance by Samuel L. Jackson. I look forward to seeing what they can do with season two. One thing I really like about Clark Gregg is that he seems just as comfortable with drama as he does with comedy; one minute he's in a David Mamet film, the next he's on a goofy sitcom, and then he's playing Leonato in Joss Whedon's version of Much Ado About Nothing.