Trying to stick to the core rules of each edition...
OD&D: The rules are confusing and form a very loose framework. The DM has lots of freedom, but is required to fill in a lot of gaps.
Holmes Basic: Levels 1-3 of OD&D described in a much clearer way.
AD&D 1e: OD&D + All Supplements and stuff from Dragon Magazine. A much more "complete" game than OD&D, but it also suffers from a lack of clarity in the rules. Also, too much was added- there are several sections should have been either revised or left out of the core rules (psionics, bards, monks, etc.).
B/X D&D: A revised and expanded version of OD&D/Holmes without most of the supplements. Clear, concise, well-balanced, and easy to learn/run.
BECMI D&D: A revised and expanded version of B/X. The C & M parts upped the complexity of the game to rival AD&D. The I portion is interesting, but rarely used.
AD&D 2e: Revision of 1e. Cleared up the ruleset and removed some of the bad ideas from 1e (psionics, assassins, monks, almost all of Unearthed Arcana). Kept/introduced new bad ideas (THAC0, poorly revised bards, unbalanced priest spheres, non-weapon proficiencies).
D&D 3e: Revised "to-hit" rolls and introduced ascending armor class, which was a big improvement. Made high-level play even less viable than in previous editions. Made "character building" a key feature of the game (selling lots of splatbooks in the process). Made miniatures-and-grid the default method of play (selling lots of minis in the process). Tried to cover every conceivable possibility in the rules (Why do this if you have an impartial DM to make rulings?). Took the fun out of being a DM (IMO). Added a broken and fairly useless skill system.
D&D 3.5e: Basically the same as 3e with a few revisions. Generated a new burst of core rulebook sales.
D&D 4e: Reinvented the game. Slaughtered many sacred cows. Made minis-and-grid virtually required to play the game. Made D&D more of a wargame/tactical rpg. New core rulebooks released every year. Didn't fix the most glaring problems with 3e/3.5e (rules reduced but are still bloated, player entitlement rampant, skills still useless, being DM is easier but still isn't fun).