It's definitely one of the most important Marvel projects out - I'd say it tops their two 'iconic' characters (as far as films go) - Spidey and IM. Why? Because with THOR you have that connection with mythology, arguably the singular aspect that the superhero descended from. It's also a bold step towards FANTASY - a step away from realism but not completely outlandish (at least, as far the premise goes). So yes, it's definitely one of the most important superhero movies coming out. Branagh was brave. I'd be hard pressed to follow up on any mythology-based films after the disaster made by his Marvel colleague Louis Leterrier on CLASH OF THE TITANS. Can't wait for this.
The First Avenger is equally important because it's arguably the first GOLDEN AGE superhero film out there. These are more than just origins for their respective superheroes, it's the origin of the superhero (archetype/genre etc).