What makes D&D unique and *the* RPG isn't just its status as the *first* rpg, it's that it is sort of astoundingly flexible.
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D&D has always had a certain 'suck it up, soldier' bent to it-- it encourages players to approach the game from a competitive stance. Beat the bad guys, get the gold, save the kingdom. You get XP for killing baddies, your characters are built to have a tactical niche, and back in the old days, you were expected to maximize every environmental advantage you could get. It is a dyed in the wool 'Gamist' game.
But nothing in GNS theory says that you HAVE to play D&D like that. You'll be rewarded if you do, but if the DM wants to run a spooky Ravenloft game where you're heroes overwhelmed by evil and you can only hope for small victories before an ignominious death, he totally can. Now, he might butt heads with some of the mechanics (HP increases, increasing power curves and the like) but nothing prevents him giving XP for story, using Fear and Madness Saves and kludging together a system that encourages his story.
Every game can be played in every way.
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As D&D has moved from 3e to 4e, the game styles it supports have gotten more and more narrow. A general system can support nearly anything.
Quantifying the range of playstyles, and making judgements about breadth or narrowness, is a bit invidious, because one's sense of what counts as meaningful difference of meaningful breadth is easily coloured by one's own preferences.
That said, I don't think D&D has particularly remarkable range. What can I do with D&D that I can't do with RM or RQ? It will handle a certain sort of dungeon crawl better, because it's hit point mechanics make frequent combats more tolerable. But RM or RQ can be pushed in this direction by adopting any of a range of mechanics or play conventions (eg frequent healing potion discoveries and friendly high level cleric NPCs). And for those who like some of the exploration components of a dungeon crawl, the tighter action resolution mechanics in RM or RQ might actually be improvements.
But when I try and use D&D to tell a sprawling tale of worthy protagonists, RQ or RM come to the fore - because unlike D&D they more naturally and readily produce richly defined PCs whose history and experiences are expressed on the character sheet - whereas it is notorious that an AD&D PC's history and experiences are often expressed on the character sheet simply as a list of cherished magic items. Now D&D can be pushed and pulled to make this sort of game work, but in my view has no special talent in that direction. Rolemaster is also a very flexible game system, and RQ perhaps nearly as much (fairly easyily tweaked to give us CoC, for example).
There's no doubt that both publishers (TSR moreso than WotC) and players have, in practice, tried to
drift D&D harder and further than probably anyone has tried with RM or RQ. But that is not a sign of D&D's flexibiity, in my view, or a cause of its market position. Rather, it is a consequence of its market position.
It's a further question whether D&D can be played coherently without drifting. In classic AD&D play, I think the alignment rules come close to incoherence - the players are expected to do their best to overcome (via their PCs) the challenge that confront them, but the GM has a standing authority to whack them with the alignment stick - but forcing the players to conform to the GM's view of what is right and wrong seems to be at odds with this whole gamist rationale. Frqeuent drifting or ignoring of the alignment rules - or use of True Neutral as the "I'm an expedient adventurer" alignment - is one result.
The tension between 2nd ed's invocations to storytelling, while presenting AD&D mechanics, are fairly evident. All sorts of drifting, plus ample use of GM force to bridge te gaps, were the result. Whatever one thinks of a game in which no combat is taking place, and action is being resolved based on diceless narration centred on NWPs and the features granted by kits, that is a long way from the game set out in the PHB and DMG.
My personal issue with 3E is that its hit points and saving throws push the game in a gonzo direction - high level wizards can withstand being bathed in fire by multiple hounds of hell even with all their spells and magic stripped away - and yet the skill rules and some aspects of the combat rules (grapple, trip, disarm etc) push in the direction of a gritty, "the real world is the limit" style of play - the same high level wizard risks drowning in water much deeper than a puddle, and is able to be knocked prone by the same hounds of hell whose breath barely singes.
For me, the latent point of incoherence in 4e is with its treasure placement rules - the suggestion is that these be treated as part of PC build, based on player wishlists - so they are not a reward but rather part of the ongoig apparatus of character development - and yet the rules suggest that they are linked to the successful overcoming of encounters - implying that they are some sort of reward for play. My own solution to this is to mostly separate treasure placement from encounters all together, and to make the rewards from encounters primarily story ones rather than treasure ones.
I think 4e is easily drifted with utterly minimal distortion to the rules and guidelines as written in either a gamist or a narrativist direction, and probably a high-concept sim/dramatist direction also (although this may require reducing some aspects of player control over PC building, perhaps by brining in earlier edition concepts of GM veto over player choice of PC build). I don't think it's easily drifted to exploration-heavy play, and that's an issue for an edition of D&D, given that exploration-heavy play is central to classic D&D, but I'm not sure that that shows that 4e is especially narrow - just that it is perhaps not especially suited to its target audience.
