While participating in a discussion online about D&D, it occured to me that each "edition" of D&D could be reasonably well summed up in a few key points or words for what it was most "known for." By this, I mean numbered editions; Original Brown and White Box D&D, or the Basic and Up lines were sidelines of D&D that had a lot of good points, but I don't believe anything that's particularly.... unique, for lack of a better word.
In all fairness, though, they weren't always that good. The art and layout was often amateurish. Gygax's prose stunk, and he needed a good editor, and someone with a better sense of organization to help him write that DMG. So many of those classic monsters were laughably stupid. So many of those classic modules were so poorly designed. And the play paradigm was strongly rooted in Gygax's own miniatures wargaming background, despite the fact that much of the audience was not.
Fourth edition: huge focus on miniatures and tactical combat. Points of light. The DDI. Massive change to the mechanical fundamentals, many of which wooed those disenchanted with the "stifling" environment of third edition, but many of which alienated other long-time players who felt like this new game simply wasn't D&D anymore. There was another mid-stream "half edition" of sorts, in the form of D&D Essentials. Also well-known for having to compete with itself, in a way, in the form of Pathfinder. Lots of internet rivalry between fans of the two editions.