I think that if Thanos were to land on Earth and setup shot (think what Daredevil did with the Hand in Shadowland) it could work. Given the lack of any real plan in the first Avengers movie besides "let's run around fighting people in the streets" I suppose a large portion of the movie would be the Avengers trying to fight through defenses and storm the castle. If you wanted something for the lightweights to do, send them after infinity gems. Surely, that would be why Thanos is on Earth, to collect the gems that are hidden there? It would make for a fast paced movie.
I agree as I can't see Marvel saving Thanos for a movie that may not even happen (Guardians of the Galaxy) and even if it does may not be for another 2 to 3 years at the earliest. You wouldn't want to have a big reveal at the end of your most successful movie only to have the character sit in limbo until a possible GofG movie. It is possible that Thanos would appear in the next Thor movie, maybe not as the main villain but just to keep him active on screen and in the minds of moviegoers. Besides how viable would a GofG movie be anyway? Don't get me wrong as I love them and was very disappointed when the title was canceled a few years ago. However how many successful Sci-Fi themed action movies have there been in the past decade? Obviously it will take a very good writer and director with a specific vision in mind like Whedon to pull it off.