There has been much speculation regarding the galactic-scale super-villain ever since he appeared in the stinger for Marvel's Avengers. First, there was speculation that he was the main villain for Avengers 2 until it was announced that the plot for that movie would follow the storyline for Age of Ultron. Some also suspect that the item Star Lord (What, you haven't heard of him?) is trying to steal in the trailers for GotG is yet another Infinity Stone. Add to that, mentions of Infinity Stones (aka the Infinity Gems in the comics) and the Tesseract (aka the Cosmic Cube) as well as the actual visual of the Infinity Gauntlet in the first Thor movie. Did you blink and miss it?
*SPOILERS AHOY*