You're surprised by this, right? No really, be honest. Totally surprised.
Let's cut to the chase: Entertainment Weekly has a big profile of Guardians of The Galaxy complete with new set photos (not yet online), and it features quotes from Director James Gunn that officially confirm what everyone has suspected/totally known since the film was announced. Brace yourselves: Guardians will definitely have a connection to Avengers 3.
Describing Thanos - first seen in one of the stingers for The Avengers - as the "head of the snake" in Guardians, Gunn told EW, and I quote, that he is "definitely connected to Avengers 3." This is obviously as surprising as the news that Thanos, the hulking, giant purple alien bent on collecting all of the Infinity Gems ("stones" in the Marvel Cinematic Universe) will be motion-captured CGI onscreen in GotG, which was also confirmed in the EW story, and we appreciate your thanks for bringing you this important news.
Gunn also gave some details into the MCU version of Peter Quill/Starlord's origins. "[H]e was abducted from Earth (Missouri) at age 8 and raised in a group of thieves and smuggles called the Ravagers," Gunn says, continuing that "He's good with the ladies and a little bit of a dick and very lucky." Meanwhile, Zoe Saldana's Gamora also has a slightly different origin. "Thanos slaughtered [Gamora's] family and Nebula's, then raised them together as siblings. They were bio engineered to become living weapons, so he's really good at killing people."
We'll be sure to share the images from EW's article as soon as they're online. In the meantime, enjoy the new image up top, which came from James Gunn's twitter. So, now that the obvious has been confirmed, let's speculate in comments about just how they'll tie the films together (aside, obviously, from the Infinity Stones quest). My honest guess is that the Guardians stinger will have them interact directly with one of the established members of The Avengers, setting them up for a return when Thanos reveals his ugly head come Avengers 3.