Lots of people have their own understanding of how D&D has changed over the years and their own opinions of just whether these various changes have been good or bad and whether any have been extensive enough to make this or that edition "not D&D."
Naturally, I have my own opinions on these matters and I'll probably get around to discussing them at greater length in due course. For now, though, I will point out one way that earlier editions of D&D were different: the art. I'm not even specifically talking about the content of the art so much as the fact that early D&D benefits, I think, from a lack of coherent art direction. Indeed, I doubt TSR even had a formal art director until many years into its existence. Certainly no such person is credited in 1983's Monster Manual II, one of the last books of the Gygax era of AD&D.
Nowadays, though, D&D is all about its art direction. Defining "the look" of D&D seems to have been a significant part of both the 3e and 4e design process. I haven't seen a lot of 4e's art, but 3e's shows a high degree of consistency over the course of the edition's run. Though there were many artists who graced 3e's pages, they all adopted similar styles in my opinion, which makes it very possible to talk about a "3e esthetic" in a specific way that you can't about 1e books, for example.
Obviously, the 3e books look very nice. They're extremely high quality books, masterfully designed and much of the art is gorgeous. Unlike a lot of self-professed grognards, as I've said, I actually like a lot of modern D&D art and find much of early D&D's illustrations amateurish at best. That said, the one area where I think early D&D wins hands-down, though, is in terms of the variety of its art. Perhaps by design and perhaps by simple necessity, there really isn't a lot of consistency in early D&D art. If you compare Dave Trampier's 1e Players Handbook cover with David Sutherlands's 1e Dungeon Masters Guide cover, for example, you see two similar but not identical styles, with Trampier's being a gritty, "low fantasy" approach to its subject matter, while Sutherland's City of Brass encounter suggests quite the opposite. The 1e Monster Manual, whose interiors were done by Trampier and Sutherland, varies even within a given artist's contributions, particularly Trampier's pieces, which range from the starkly realistic to the cartoonish.
I don't think early D&D art is better, either technically or esthetically, than more modern treatments of the same subject matter (though I am very fond of certain older pieces). In almost every way, I think modern gaming art is superior to its predecessors. The sole exception is in its implication. This is a subtle and subjective thing but, for me, early gaming art, in its diversity, with its rough around the edges rawness, is far more evocative of what I want roleplaying to be -- an amateur activity -- and better highlights that it is an intensely personal thing that varies greatly from gamer to gamer. Frankly, I don't want slick; I don't want polished. Roleplaying games grew out of the confluence of multiple kit-bashes to wargaming in the late 60s and early 70s and it's that gung-ho do-it-yourself enthusiasm that I see in early D&D art.
It's that same enthusiasm I just don't see in many modern gaming products, particularly D&D.