In a new interview with Empire Magazine, Marvel Studios president Kevin Feige has confirmed the main villain to be actor Lee Pace's Ronan.
"Ronan the Accuser is very much the main bad guy. And Thanos is lurking above it all. You will learn more about Thanos in Guardians, for sure. Certainly you'll get more than the one turn around and smirk. You'll get much more than that!"
Feige also talked about how Guardians of the Galaxy will be connected to the Marvel Cinematic Universe.
"We've already said that Thanos plays a part in it and is a character in it. That in and of itself should tell you it's connected to the other worlds."
Ronan has already been previously confirmed to be Thanos' general so it seems like he might be hunting the Guardians to "bring them to justice" per Thanos' orders. Just a guess though and we'll have to be patient until Guardians of the Galaxy releases in theaters on August 1, 2014 to find out.