I'm not worried about the look per se (and I like that they're giving him the helmet). I'm worried that cutting his hair and destroying Mjolnir signals that Thor is going to be somewhat "mortal," for lack of a better word, for a significant chunk of the film. I hate when superheroes lose their abilities in movies. I don't want to see Superman get beat up in a diner (or drinking his face off), I don't want to see Spider Man falling off walls, I don't want to see Professor X unable to read people's minds.
I'm sure there are probably a few exceptions where I've enjoyed stories of superheroes who lost their powers, but generally, I go to the movies to see the superheroes be super, not to see them be human. Those kinds of stories are more compelling in the books than they are in a 2-hour film where the hero only has his powers for about 25 minutes.
I think he may lose some powers (flight and thunder stuff), but they show him throwing down with the Hulk sans Mjolnir. The hammer is just a portion of Thor's powers. His strength and durability are mostly his Asgardian heritage. I wouldn't be too worried about him coming across as mortal.