Of all the films leading up to The Avengers, I loved Iron Man and Captain America, tolerated Iron Man 2, disliked The Incredible Hulk, and hated Thor. My problems with Thor stem both from the source of his power and his lack of heroism. I can buy into Iron Man because Tony Stark is a billionaire genius who's been designing weapons all his life, and I can accept Captain America and The Hulk because both Steve Rogers and Bruce Banner were transformed by medical experiments intended to create super soldiers. But Thor is a god from another planet (who just happens to look human and speaks English with a British accent despite being a Norse god), which is illogical and doesn't fit with the others. Further, while Iron Man and Captain America actively desire to protect others, The Hulk and Thor aren't at all heroic and only fight when they themselves are attacked. I was truly disappointed to find that The Avengers not only focused heavily on the supernatural storyline initiated in Thor, but that it also dragged the others further into its absurdity.
The film opens in S.H.I.E.L.D. headquarters, where the mysterious Tesseract cube (which had been recovered by Howard Stark after Captain America sent it into the ocean during a battle with Red Skull) resides. The usually well-behaved cube has recently been acting up, and that's because Loki (Thor's adopted brother turned nemesis) is trying to use it to open an intergalactic portal so that he can invade with a newly-acquired alien army. Loki's attempts are successful, and not only does he steal the Tesseract from S.H.I.E.L.D., but he even manages to use mind control to turn a couple of them (including Hawkeye and Dr. Selvig) against the others. If he succeeds in using the Tesseract to create a portal for the alien invaders, then Earth is done for.
Fortunately, Earth is not without protection. Captain America is more powerful than ever after having been thawed out of his arctic coma, and Iron Man is his normal arrogant self, but with the technology to back it up. The Black Widow also gets in on the action and helps to track down Bruce Banner. They don't really want The Hulk, but Banner's expertise in gamma radiation may be the only way they can find the Tesseract and the bad guys who have it. And of course Thor isn't quite as stranded on his home planet as everyone (except of course for the audience) seemed to think. But even with their combined resources, along with the old standbys like Nick Fury and Agent Coulson, it's not going to be an easy fight.
The Avengers isn't a bad movie, and it does some things well. The Hulk is much more well represented in The Avengers than he was even in the whole movie dedicated to him, and although they can't manage to find someone who wants that role for more than one movie, Mark Ruffalo gave a great performance. Similarly, Black Widow has a much bigger role, and her fighting style has become much more fun than the "strike a pose" approach taken in Iron Man 2, although it seemed out of character for her to appear terrified of The Hulk when nothing else seemed to rattle her at all. It's also fun to see the characters working together and overcoming their differences to fight a common enemy, but it's unfortunately also the case that they spent far too much time quarreling amongst themselves.
But there are also a number of disappointments in the film. There are times that it feels too much like The Transformers, with a race of alien robots heading toward Earth in search of a cube of unlimited power, and the technology they use in their fighting (and for everything else) is pretty unimpressive. Natalie Portman's Jane character (sadly one of the best parts of Thor) is completely nonexistent, except for a single mention of how she's been stashed away to be kept safe. Hawkeye apparently has the bow-and-arrow equivalent of an old west six-shooter that never runs out of ammo, until it does. Nick Fury is still almost completely worthless, as we sadly realize that he is basically middle management and not someone likely to do awesome things, and even see him mislead and lie to others in order to manipulate them. Similarly, the newly-introduced Agent Hill (played by Cobie Smulders) is pretty but completely inconsequential. Some of the action and dialogue is quite predictable, and if you know what's about to come then seeing it happen just isn't nearly as much fun. And while there are a number of very illogical elements in the film's plot, they're in areas of the film that I can't really discuss without getting into spoilers.
Ultimately, my disappointment stems just as much from the film not living up to my expectations as from it being a weak movie. I had extremely high hopes for the film based on Joss Whedon's involvement (with my excitement even higher after his recent The Cabin in the Woods), and based on the over-the-top excitement from those who'd already seen it. It would have been hard for any film to meet the expectations I had set for it, but I think that The Avengers has enough legitimate flaws to justify my mediocre-to-decent perception of it.