I've said many times that as the game advances through the levels, it literally changes genre before your eyes. This, of course, wouldn't have been surprising at all if I'd remembered and connected the dots that it used to be much more explicit. My first game of D&D was with OD&D, but I didn't really "get it" for some time. It wasn't until sometime in the 1981-3 window that the Moldvay B/X sets of D&D were out that I really got into the game.
And within relatively short order, Moldvay was replaced with the Mentzer boxed sets (y'know, with the awesome Larry Elmore dragon art.) This version of D&D is usually "acronymed" out as BECMI--Basic (levels 1-3), Expert (4-14), Companion (15-25), Master (26-36), and Immortals (transcended levels altogether and advanced by divine rank.) And each of the boxed sets indicated by a letter in the acronym represented a specific tier, if you will, of gameplay.
And although my memory of those boxed sets is pretty hazy (besides, by then most of my friends had made the switch to AD&D anyway, and I don't recall that I ever exactly saw all of the BECMI run or read through them myself) it seems that each tier focused on a different type of adventure. BECMI moved you from some apprentice dungeongcrawling, to wilderness exploration, to kingdom building, to plane-hopping, to mingling with the gods. Later editions collapsed the tiers somewhat, and also erased their distinctiveness as exppressed through modules. Basically, they were all stuck in the Basic mode, even if you were actually fighting Orcus in the dungeon instead of orcs.
These are, as I've said, fundamentally different play paradigms, different genres; almost entirely different games. But that was also kind of explicit, given that they came in separate boxed sets that you had to buy separately.
Although my own interest is specifically rooted in the basic and expert tiers (hence my preference, if I were to ever revisit an older edition of D&D, for the B/X version) I don't begrudge the other tiers their existance, and I can imagine wanting to play in any of them from time to time.
But the tiers got kind of muddled during the d20 days, and while 4e had tiers, they seemed kind of arbitrary, and there wasn't anything specific about belonging to one tier vs. the other. This isn't necessarily a horrible thing (I'd hate to think that wilderness exploration could only be done during the Expert tier, for example) I also think that focusing on tiers, instead of having the entire panoply of D&D spread out in one volume that's trying to do it all at once, isn't necessarily a bad idea.
I wonder, though, if keeping the tiers more sharply defined, and more "separate" feeling, that there can again be a sense that levels aren't a drain on the system; a sacred cow that is dubious in today's world, where level-less systems are otherwise more or less the norm.
Otherwise, the concept of Bounded Accuracy, which I described earlier (and which is also the subject of a developer post) keeps the tiers from being as noticable, because it makes the power curve of D&D significantly flatter. I'm not sure which is "better" except from the standpoint that I recognize my own tastes aren't necessarily exclusive to the market, and even if they were, even I don't want to feel like the first two tiers are my only options all of the time.