After watching Avengers for the third time it hit me.
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Loki is the only thing I dislike in Avengers. His motivation and goals are so idiotic and childish I just want to punch him. And I know that he's a stereotypical comicbook villain, so I shouldn't expect much in terms of depth, but in comparison to how well developed Joss Whedon made the other characters, he seems a bit weak.

I think Loki would've been more interesting and closer to his mythological source, if he didn't want to rule Earth. If instead he simply wanted to wreck Earth for the hell of it and because it would make Thor squirm. He shouldn't have been bound by a need for adulation and power, it weakens him. Makes him vulnerable to attacks on his ego and his logic. It also ruins the image of him as a Trickster (though I think the Thor movie solidified that). Tricksters don't crave political power, they operate against it, testing established boundaries to see how much they can get away with.

As loathe as I am to say it (because there are flaws with this character too), I think he needed a bit of Ledger's Joker in him. Mostly the self awareness, the lack of reason, and the general apathy towards the end result. As long as lives are lost and shit gets broken, he's fine, and that's what makes him terrifying. If you added that indifference with Loki's power, you'd have a scary ass villain.

With all that said though, Avengers is still my favorite movie of all time and it was REALLY gratifying seeing the Hulk bash his stupid ass into the pavement. So maybe in that respect Loki was the perfect villain. An annoying prick you just want to see lose.