Firesnakearies said other RPGs change very little. Mike says that grognard capture explains this.
But grognard capture causes RPGs to change a lot. That's the whole point of grognard capture. The grognards take over and move the game beyond its core, doubling the size of the rulebook, making it increasingly complex, and likely (but not always) making the game less accessible to newbies.
I think I can tell that what was actually meant was: a) other RPGs have grognard capture, which causes the RPG to preserve vestiges of their earlier editions, and b) D&D has been brave enough to discard un-fun vestiges and "refine" itself.
I thought about this a bit yesterday, and something clicked for me. D&D does rely on grognard capture, but it tries to channel it into newbie friendly avenues.
Grognards want all the cool stuff they had in earlier versions of the game, along with more new stuff to play with. That's the root of the ever growing core rulebook.
D&D cheats this by redefining its rules. That gives grognards the fun of playing with all sorts of new stuff, while also keeping rules bloat down. Rather than add new rules to the game, D&D's new rules replace old ones.
Now, that runs the risk of alienating people who liked the old rules just fine and who don't like the new rules. If you do a good job, though, you make people on both sides of the fence (newbie and old guard) happy. Of course, you can never make everyone happy, but your goal is to make enough people happy.
Now that said, an update that keeps the core intact is fine, but I think it's bad in the long term. If you look at 3.5 compared to 3.0, folks adopted it pretty much wholesale. You don't see anyone making a 3.0 game a la Pathfinder. A new edition that follows that model can work, but I think it's more of a short term strategy.
Looking at 3.5, you see the seeds of added complexity - weapon sizes (every weapon now has seven versions), buffs measured in minutes rather than hours (players have to recalculate stats on the fly from encounter to encounter). If you can't change the core, but you do want to change something, by definition you have to add rather than subtract rules.
Of course, all my theorizing overlooks the inherent contradiction of producing editions that rewrite rules. Good businesses listen to their customers, and you could easily argue that many of D&D's customers would want an incremental, compatible update.
When I think of that, I remember an anecdote I picked up back during the dot.com bubble. Back when the minivan was on the drawing board, the sales execs thought that it was a terrible idea. All their data showed that customers wanted a better station wagon. Of course, the minivan proceeded to eat the station wagon alive.
The point of the story is that people ask for what they know. They didn't know what a minivan was, so they couldn't ask for it. But it also means that you have to be thorough and confident in such a change. Hence, both 3e and 4e after it went through rigorous, long development and playtest cycles.