I have to say I'm impressed. I don't really have confidence that the movie will be that good, necessarily (especially compared to something like The Dark Knight Rises, which doesn't have to find a way to get so many high-profile superheroes together and give them something credible to do), but it certainly looks cool. Really, really cool.
There are some cool moments in the trailer, and I have no doubt it will be fun, but really there's a bit too much of TRANSFORMERS in that trailer for my liking. Though, I'm still inclined to trust Whedon at this point. At least, unlike some of the other Marvel films so far, it looks like they'll actually be facing a threat that should necessitate the whole Avengers team.