Warning: Contains Spoilers
(so no blubbing that I ruined it for you if you read any further)
1). The Avengers beat the shit out of each other almost as much as the bad guys (Thor vs. Iron Man AND Captain America, Black Widow vs. Hawkeye, Hulk vs. anyone within reach.
2). Film is certified 12A but Loki calls Black Widow a "mewling quim". I predict many kids being told to shush by their parents on the ride home as they innocently re-enact that bit.
3). For a kid's film there are many hidden bonuses for adult, heterosexual males. These include lots of shots of Scarlet Johannsen's perfect arse clad in tight, black Lycra; Potts whispering something filthily perverted in Tony Stark's ear and Black Widow getting tortured in just her bra and a skirt (not to mention kicking fuck out of her captors while still tied to a chair).
4). Loki has a hissy fit at the Hulk for not kneeling to him. Cue the most violent beating I've ever seen in a kid's film. Even though Loki's a demi-god, it takes him the remainder of the movie to recover from it.
5). Hulk being stronger than Thor, but totally unable to lift Mjolnir no matter how hard he tries.
6). Hawkeye killing dozens of good guys while possessed, but everyone forgives him as it wasn't his fault.
7). Perfect casting of Samuel Jackson as Nick Fury, the Avengers' boss. When HIS bosses overrule him and launch a jet to nuke Manhattan island, he simply shoots the plane down with a hand held rocket launcher (fate of pilot never fully revealed).
8). Cameos from Powers Boothe, Jenny Agutter, Harry Dean Stanton, not to mention the immortal Stan Lee.
9). The death of a secondary but much beloved character.
10). Tony Stark being thrown off a roof and being followed down by his mark 7 Iron Man suit which he gets into in freefall and then jets away.
11). THAT shot of the Avengers stood in a circle with Hulk roaring, Black Widow reloading her pistol, Hawkeye notching an arrow, Captain America hefting his shield, Thor raising Mjolnir and Iron Man descending from the sky to stand amongst them.