Well, that, or maybe Avengers is just a fantastic popcorn flick and nothing more (or less).
I'm not saying Watchmen is all that profound or a contender for best movie ever - I just think it was deeper and more interesting as something to think about after the experience is over.
Everything I've seen from Whedon has been superficial and about snappy dialogue above all else. This includes Firefly (which I happen to really love) - but there's no reason to pretend it's something more than it is.
Well, I don't think there's a good reason for that.
Oh, and I like Batman Begins mostly because of the craftsmanship - and I don't think it's dark or depressing. It's surprisingly mature given the material, though - and I think it has some great scenes in it (like the hallucination stuff with the horse and what not).
I don't think much of Dark Knight, however.