D&D has been used for multiple modes of play outside of its designed parameters thanks to its popularity and place in the RPG community. Thanks to the larger number of players and name brand of D&D, its often easier to put together a game where you use D&D against what its designed to do than it is to play a different RPG that does what you want. At its core though, and in its design, D&D is a down the middle game for the masses.
AD&D was in practice a very rules-light game. There were books filled with fiddly and arcane rules, but these were easily ignored or houseruled in practice. The amount of core rules the game contained that couldn't be ignored or changed was actually pretty small. This core of the game was fairly independant of the fiddly rules for various things and playstyles that the game itself possessed, and existed outside of custom rules created to make D&D do things it wasn't designed for. The end result is that the core game didn't suffer for this versatility.
As far as D&D being used to play styles that it wasn't designed for, 3E embraced this, and tried to be flexible and gave nods to a vast variety of playstyles. In my opinion, trying to be flexible and accomodating varying playstyles weakened the center of the game and made it a weaker game in terms of D&D's core audience. The combination of 3E's attempt at being able to do anything combined with the OGL movement also had a very negative effect on games that were not D&D or based on the d20 mechanic.
4E returned D&D to its roots and focused on its core audience, concentrating on doing the core D&D experience as well as it can be done while downplaying the ability of the game to be used outside of its core focus. In my opinion, this is a good thing, as people who want to play beer and popcorn, action/adventure, kill the monsters and take their stuff D&D get a game better focused on doing that, while non-D&D(and particularly non-d20) games that offer something different don't have to compete with D&D trying to achieve the same thing only not as well but having a bigger brand name.
Some people who really liked the versatility and customizability of 3E D&D are losing out, but things change and you have to deal with it.