This announcement really shouldn't surprise anyone who saw 'The Avengers'. The snippet of Thanos at the end of that film signaled Marvel's intent to push into the "Cosmic Marvel" universe.
The choice of this team may be a little surprising however. The Guardians have been in existence since 1969, but didn't get its own comic title until 1990. And there have been many different members of the team since its inception. The most recent of which (seen below) seems to be the lineup set to appear in the film. I think it's funny Marvel didn't choose a more recognizable name by going with Captain Marvel, or Adam Warlock, or Nova, or Quasar.
Official concept art for the film revealed at Comic Con
Those with keen eyes may notice a tree and a raccoon in that picture. They would be correct. Two members of the Guardians are Groot, a living tree, and Rocket Raccoon, a raccoon.
To me, this illustrates the wide gap between the Marvel/Disney partnership and the DC/Warner Brothers partnership. We will see a movie featuring tree and raccoon superheroes no one has ever heard of before we see a big-sceen Flash or Wonder Woman. I could go up to nearly anyone on the street and they would be able to recognize Flash and Wonder Woman, and probably be able to give me a basic idea of their powers. Yet the only characters DC seems willing to adapt for the big screen are Batman and Superman (currently filming another reboot). And so far their other attempt at a major hero (Green Lantern) was a failure as a comic adaptation and movie both. I can't stand the amorphous space blob villain idea. It never works story-wise and looks lame on film. (I'm looking at you Fantastic Four: Rise of the Silver Surfer - Note: not a Marvel Studios film)
And Marvel to this point has had several successful character franchises (Iron Man, Thor, Captain America) where DC/Warner Bros has only had one (The Goddam Batman).
It's disappointing. It seems like Warner Bros. is afraid that any characters outside of Batman and Superman won't have a market. Meanwhile, Marvel/Disney is willing to roll the dice with a C-list team and will probably find success. Doubly disappointing since 7 times out of 10 when I go to read a superhero comic (trade paperbacks because I'm cheap and impatient), it's DC.
Marvel also announced dates and titles for Iron Man 3, Thor: The Dark World, Captain America: The Winter Soldier, and Ant-Man. All but Iron Man 3 are years away, but Marvel clearly has a plan. It is rumored that DC will be making a Lobo adaptation with Dwayne "The Rock" Johnson. But, the project has been stuck in development hell since 2009... Lobo. Now I like Lobo just fine, I just cant figure out why they would go with Lobo over a more well known character.
But hey, I still have high hopes for Superman: Man of Steel.