B- for me.
I don't know, maybe I was just older than the target audience, but I thought that the plot was very thin. Actually, describing it as "very thin" probably is too kind.
It's basically "bad guy wants to conquer the world, good guys stop him, MacGuffin thrown in."
A lot of needless fights for the sake of fights(Hulk vs. Thor, Iron Man vs. Thor, etc.) and the last hour is pure, numbing action that ends up being boring.
The best part was on the ship with Loki captured, because there was actual dialogue and good character stuff. Still, some funny lines, it was decently entertaining, it wasn't bad, just very meh.
The Iron Man movies and Thor movie were better than this.