I think the success of Thor was with two very key aspects. The first was downplaying him as a "god." I didn't see how that would work in a "real life" world and what they did, "mankind has a habit of deifying that which they can't understand" was perfect. The second was casting Kenneth, a Shakespeare nut, to do the film. Unfortunately, what kept it from being truly epic was what wound up on the cutting room floor and with him gone, I have little faith in the sequel.
Not to sure. They know there are several franchises depending on the success of The Avengers. So I don't think they'll tread lightly.
A blessing in disguise too is they have Disney's pocketbooks to reach into. And I think they'll utilize that here as the grand scheme of things calls for an epic action movie, not campy coffee shop dialogue between characters. And as Carl and Mike have already mentioned, this is where the worry about Joss comes into play and I agree, it's entirely warranted.