5.) Thor just appears right the eff out of nowhere.
No, seriously. He just suddenly arrives to inform Loki that he's Grounded For Life. Last we saw him, he was sad because he couldn't get back to his girlfriend. But wait, apparently Papa Odin has some way to send him to Earth! Loki mutters something about dark energy, but everyone was too distracted by his cape to pay attention.
Oh. That explains it.
4.) The Hulk suddenly becomes a team player.
Part of Loki's Evil PlanTM was to unleash The Hulk on Nick Fury's flying...boat...thing. It worked quite well, and we see Bruce Banner sitting naked in a heap of wreckage where he fell from the flying...boat...thing. He looks quite sad, and is not even surprised that the old security guard is apparently unaffected by large green men falling from the sky. In the Marvel universe you get used to these sorts of things. If you survive at all, you're automatically upgraded to "Action Survivor". Bruce is sad, because he nearly killed the Hot Chick on board.
Then, suddenly, as the rest of the team are all assembled (LOL SEE WHAT I DID THERE), Bruce arrives on a tiny motorcycle, and suddenly the Big Guy is all ready to help the Avengers. What did Bruce do? Did he and his dark side have a heart to heart? Did he promise the Hulk great smashing fun? What could have been a fantastic scene of Bruce fighting his dark side is apparently not as important as things blowing up.
"Dear Princess Fury, today Hulk learn lesson about friends..."
3.) Why is New York always destroyed?
And more importantly, where do they get money for the clean-up? I mean seriously. At least Preston and Child have small grade disasters that don't wind up destroying every skyscraper possible. And yet they manage to miss any important buildings, which makes Roland Emmerich cry.
2.) MECHANICAL SPACE TURTLES.
With tails. COME ON CHITAURI. At least make your ships look scary. Look at the Shadows. Why don’t you take a lesson from them? No, wait, don’t. You might actually be effective villains then.
"So Tony, you were like 'Whoa!' and we were like 'Whoaaaa!' but then you were like 'Whoaaa.' CHA."
1.) Loki’s hat.
I DON’T CARE IF IT’S FROM THE COMICS, YOUR HAT IS STUPID LOKI. IT’S JUST STUPID OKAY?
This image pre-captioned for your convenience.