Here's something scary - when you think about it, the D&D evolution is a lot like the evolution of Windows...
oD&D is a lot like Windows 1-3.1 - new, not too flashy, but gets the job done (albiet with a lot of tinkering for more complex jobs).
Basic D&D (edited by Holmes) codified things a bit and more importantly made things more accessible for the consumer - a lot like Win95
AD&D was the NT of its time - more complex code than its "lite" cousin, it was built for people who like to beat the heck out of systems. It also was more unfriendly to change (remember Gary's rants in the old Dragon mags about D&D and chess?) and notoriously difficult to tweak.
The next two Basic sets (by separate editors) were largely repackaging of a successful product, and meshing the different basic sets is fairly easy - they are extremely backwards compatible (just like the various 9x systems - if it ran on 95, odds are it will run on 98 or 98SE).
Meanwhile, the addition of Unearthed Arcana was the NT SP4 of AD&D: lots of consumers stayed with SP4 for quite a long time, even after the next updates came out (SP5, or for AD&D the players options) because it proved to be fairly stable, if quirky.
3rd Ed is much like Win2K - it started trying to bridge the gap between the two systems (and 2K, and its successor, XP now claim to be a platform that will encompass everyone's needs, just like 3rd Ed does).
Which brings us to 3.5 and XP - the latest offerings. At their core, they both use basically the same kernal as their predecessor, with a few tweaks here and there. And both, by the way, come in a slick new package.