I really don't think much marketing is going to be required for Avengers, though I am sure there will be a huge marketing blitz come March of 2012. The fact is, IM and IM2 were very successful, Hulk was a modest hit and is instantly recognizeable and I am sure that Thor is going to be a huge movie. The GA knows of Thor, I heard people talking about him during the IM2 post-credits scene, though they were like "who's going to play him? Dolph Lundgren?"
Cap is the real lynchpin here, it is the last opportunity before Avengers to put these characters in position to interact with each other. The movie has to deliver and I think it will. The last scene of Cap needs to foreshadow the Avengers and I think that is why Whedon is touching up the Cap script. He wants to make sure that these movies are in lock-step with each other.