After the release of Iron Man 2, which truly began to set the wheels for The Avengers, they still had to bring in other superheroes of course. They had already done an "Incredible Hulk" movie, but due to a re-casting of the Hulk, no one really knows if that movie is canon or not. So this was really the first movie in the Avengers series to feature a hero other than Iron Man... with a cast member who actually stayed on.
Much like Iron Man, Thor takes us into somewhat more obscure territories of Marvel. Norse mythology in the Marvel universe? Seriously? Well, they had to set up the sci-fi part of the MCU somehow since they weren't ready to give us Guardians of the Galaxy yet.
Thor does indeed center around the God of Thunder and his large hammer with insane power. While he hails from Asgard--another "realm" entirely, also from Norse mythology--much of the movie actually does take place on Earth. Why? Because Thor--heir to the throne--does something really stupid and starts a war. Early on in the movie, he's both arrogant and a bit battle-hungry. So his father Odin strips of him his power and banishes him to Earth.
So thus he is stuck on Earth, forced to live without his power and to learn a few life lessons and be confused by Earth customs. Meanwhile on Asgard, there's still trouble as Thor's brother, Loki, wants to stir up some trouble of his own. Unfortunately the original trailers never even tried to hide the twist of Loki being the main antagonist--even if it's not that shocking anyway.
While Thor may throw us quite a bit of Norse mythology--which many may be unfamiliar with--and does some more universe-building, it's actually fairly simplistic. It's actually not that different from Iron Man's origin story in that the titular character ends up in a situation that makes him rethink life and have to become a better person. The only difference maybe being that Thor takes more strides--he's more obviously a hero and changed man by the end of this movie (not to discount Tony Stark's redemption).
Now one thing that might seem strange to some is that this movie is directed by Kenneth Branagh. A guy who's mostly known for his Shakespearean film adaptions doing a superhero movie? He actually pulls it off quite well. We're given mostly likable and memorable characters--from Thor himself and his love interest Jane Foster to even the antagonist Loki, who got a good start to his long tenure as the most memorable Marvel villain. It's a well-made movie with strong action sequences and special effects and good pacing. The story, even if predictable, is well done too. There's also quite a strong cast, including Chris Hemsworth, Natalie Portman, Tom Hiddleston, Stellan Skarsgard, Idris Elba and Anthony Hopkins. Plus Samuel L. Jackson in another post-credits scene.
While Thor may not be among the strongest of the Marvel Cinematic Universe pack, it's still quite good enough to stand on its own. It's noteworthy in that it was really the first big risk for Marvel Studios--it could've easily misfired--but they pulled it off quite well and continued to set up future movies too. It may not be truly special, but it's still as good a superhero film as any.