^ Well, of course it will be silly from a certain point of view, but no more or less so, as far as I can tell, than a lot of other movies of its kind that I have enjoyed. If it has a sense of humor about itself, which apparently it does, then so much the better.
Beyond that, it is possible to tell a grounded story about characters with very human traits and flaws that also contains a lot of fantastical elements, so really the outlandishness of the Thor mythos (which is indeed a very wacky and rather odd mixture of fantasy, sci-fi and mythology even for comics) shouldn't stand in the way of the film telling a rousing story. Branagh strikes me as the perfect man for the job, so I'm looking forward to this as well.
As for the posters, meh... I think it's just the nature of image saturation in pop culture that leads to very simple design, with a face, a name, an intense color, maybe a phrase or two, which is all the casual audience of moviegoers is likely to retain from a poster a few months prior to the film's release (in the opinion of the designers in any event).