Mercurius
Legend
I grouped these comments together, because they all, for me, demonstrate the same phenomenon. They all assume a focus - a raison d'être - for D&D that I think is a valid possible focus, but not the one 4E uses. I'll return to the "one 4E uses" bit below, but, Mercurius, you are assuming that "immersion", character building based on concept derived from the campaign world and the system being "more realistic" are "Good Things" (TM). While there is nothing terribly wrong with any of them, I don't think they are either necessary, nor the best focus for a game that has character levels and hit points.
I agree with you that these elements don't have to be the focus of D&D, whether in the form of 4E or not, let alone all RPGs. However, I think D&D should at least be able to accommodate multiple styles and foci. The d20 engine is certainly flexible enough to handle just about anything one can imagine.
Another view is that the approach that I'm advocating--which theoretically produces an experience of greater immersion and connection to the imagination space environment--leads to a more satisfactory experience that one would want to come back to, versus one that is less immersing, more focused on "being fun," because it offers something (immersion within a shared imagination space) that no other related activity can quite manage, certainly no competing activities such MMOs and board games.
Ideally we can have our cake and eat it too, right? We can have a game that appeals to as many people as possible yet also is able to focus in on specific styles and different play experiences, all while capitalizing on the strengths and unique qualities of the medium. I would love for D&D to accommodate, for example, "off the farm" and gonzo fantasy; quick-and-deadly combat, but also intricately detailed tactical maneuvering; hundreds of spell options but also spontaneous casting. And so on. And of course the caveat should be, imo, that the primary "game table" is the imagination space, never something tangible with the senses. As soon as the sensory becomes the primary game space, it stops being an RPG in the traditional sense.
Now one could argue that most cutting edge RPGs have a rather tight focus; that the most aesthetically pleasing and sophisticated RPGs tend to be either tightly designed mechanically (with the common word being "elegant") and/or fused to a particular theme or setting. But D&D is meant to be played, not read, appreciated, and maybe tried out a few times before one moves onto the next Forge-born creation.
D&D is, and probably always will be, the most played RPG in the world. It should be (imo) both very simple at its core and capable of handling a substantial degree of complexity. And it should, as I said, retain the focus on the imagination space over the battlemap or any kind of virtual simulation.