To tie into my posts up thread, I find the Mentzer Red Box special because of its marketing and placement. Finding it in my local book store (Walden's I think) was like a blast of fresh air. Not only was this a game that threw off the shackles of predestined stories, but it was also something I could share with friends. We could make our own stories.
I would think that in today's age, selling an experience like D&D would be easy if they would approach it like: this is a game that does not rely on a group of programmers to predetermine your game or story, your game is limited only by your imagination. This is D&D's strongest selling point. By far.
That's why the Red Box is special to me. It was really unlimited in scope, if not in rules.