Still, in this one, I could never quite figure out what exactly it would take to hurt our heroes. Iron Man, I have to assume, must have a lot of de-gravitational inertia-absorbing uber-padding in his suit, because the body slams he takes inside the armor should have turned poor Tony Stark into jelly. Actually jelly would probably have more consistency than what would be left after some of these battles. In case this doesn't make sense to you, try this handy experiment at home. Put on a suit of armor, as heavy and thick as you like, and let a bus run you down. Hell, let a Mini-Cooper hit you at 12 MPH. Now explain the result to your medical insurance provider, or, more likely, let your puzzled and bereaved family explain what happened to a funeral director. I smell YouTube video all over this one.
Thor and Loki I guess are protected by Asgardian magic, but a strong enough wallop can still cause momentary dazedness. When Thor's sent plunging "30,000 feet" inside a specially designed containment unit, there is genuine concern that the impact will kill him. But (SPOILER!) he busts out at the last second and makes it out okay. However Loki seems especially tough. Toward the end of the film the Hulk body-slams the poor guy like a rag-doll, whamming him into cement about a dozen times. Post-slamming Loki is half-buried in the floor and issues a semi-comic wheeze, but the next time we see him he looks none the worse for wear, save for the obligatory cut lip. Evidently the lip region doesn't get the same protective shield as the rest of the Loki carcass. Perhaps there is some Asgardian chapstick that could alleviate this condition.
The Hulk is just... really friggin' tough. I have to say, this is the first movie that has really got him right. Of course I only saw the Ang Lee Hulk and not the the other one, so maybe he was right in that one too. He was not right in the interesting but "what the fuck?!" Ang Lee movie. But he's super-right in THE AVENGERS. The Hulk can basically take just about any punishment, though concentrated blue-energy fire from a bunch of aliens appears to at least drop him to his knees. Still, I get the feeling that Earth could be a cinder and the Hulk would still be punchin'. Of course me like Hulk, so he gets the benefit of the doubt.
Captain America is just really, really tough. An alien energy bolt finally cuts his tummy (he makes an ow-ie gesture for about a minute), but otherwise he seems able to take an enormous licking and keep ticking. And that shield, wow. Full bore alien bolts of blue lightning just fly off, with very little reciprocal bounce-back on the part of Cap. Here's another experiment you can try when you're allowing buses and cars to hit you in your armor. See how far you fly back upon impact. There's a scientific law that explains this involving inertia and stuff, but I'm having a couple shots of Gray Goose orange vodka as I write this and can't really get up the energy to do the research. But you can trust me on this science-ey stuff -- I worked on Battlestar Galactica! Oh, and yes, you can brace yourself for the impact, but believe me, all that force has to go somewhere. See reference to Tony Stark and "jelly" above.
Hawkeye and Black Widow are basically just dudes who are really strong and smart enough to not let Loki sneak behind them and spear them. I'm just sayin', "Phil," you gotta keep your eyes peeled, buddy! Well, actually (oh shit, SPOILER), not anymore. Or maybe not. This IS comics we're talking about!
I don't know what can hurt Thanos (the big blue-ish guy who appears for a moment after the credits), but man does that guy have nice teeth. I suggest the Hulk go for the guy's choppers, clearly a source of vanity for the otherwise alien, blue-skinned, lizard-textured galaxy-smashing future villain. Or maybe I'm misreading that scene... I can see it now. "AVENGERS 2: THANOS BUILDS A PETTING ZOO." Hey, admit it, nobody'd see THAT coming...