OD&D and BECMI D&D were rule guidelines to be used by DM in creating their own fantasy adventure games. Any part of the rules could be tinkered with, and whole new systems substituted for the ones given, all without fear of "breaking" the game. There was no campaign world integrated into the rules, and even later when the "Known World" of Mystara came along, it was easy to ignore.
AD&D essentially grew out of Gygax's own OD&D games. Despite his largely ignored call for a certain amount of orthodoxy (done to promote the RPGA), and his inclusion of a very small amount of material from the Greyhawk campaign (some spell names and artifacts in the DMG), AD&D was not only easy to house rule, it pretty much required house ruling to run! The DM really "owned" the system, and the game could vary wildly from table to table.
2nd Edition was just AD&D with all the Gygaxian charm and quirkiness taken out. It played pretty much the same way, and was still very easy to tinker with.
3rd Edition promised to be a faster, more streamlined system. The problem was that almost every rule in 3rd edition references every other rule. While the intent seems to have been to make the game more "logical", the effect has been to make it a total system, much like Marxism; either you accept it all, or you don't accept it at all. Tinkering with 3rd edition quickly becomes an exercise in frustration, as any minor change cascades through game play to create numerous unforeseeable problems. Despite its post-Gygax Greyhawkisms, however, 3rd edition is still essentially a generic game.
4th Edition does seem like it is a complete package, of rules and implied setting wielded together into a singular entity. From what I've seen, it doesn't look like the new game will have any place for DM who like to tinker with the game system, or run games with a different "flavor" than that implied in the rule book. By moving flavor and campaign setting into the Players Handbook, it looks to discourage DM creativity in favor of uniformity from game table to game table. For instance, it looks like specific places will be referenced in the Player's Handbook, monster will be given specific default histories tied to the implied setting, etc. 4th Edition might be a good game, although it doesn't look very much like D&D to me. What WOTC seems to have done is created a new fantasy roleplaying game, and slapped the D&D label on it in order to leverage their brand. In the process, it looks like they've destroyed some of the fun of the older game; DM's exercising their creativity and imaginations.
The D&D branding is so strong that WOTC probably will initially make a profit on the game. Ultimately, however, I think 4th Edition will be considered a failure, and will probably be the last tabletop version of D&D before MMORPGs completely destroy the hobby.