I'll agree that the rules themselves have a feel to them. I, for one, never did play in any of those "classic" adventures except for Undermountain (with which I had an experience similar to Alphastream's experience with Keep on the Borderlands), yet each incarnation had a different feel to it because each incarnation had a different approach to the same problem. A prime example of this is with the way AC differs between 2nd AD&D and 3.0.
It seems to me that any attempt to recapture the "classic" D&D feel with "modern" rules comes down to where the simplicity and the complexity come into play within the system. Look at 2nd edition AD&D as an example: When it comes right down to it, AD&D was really a simple game in spite of the A in its title. Sure, none of the subsystems could agree on a core mechanic, but each was simple in it's implementation. The complexity entered into the game with the equipment (both magical and mundane, the equipment tables had four flavors of footwear for Heaven's sake!) and the magic. Compare that with 4E, in which the core Fighter class occupies 14 pages of the Player's Handbook while the treasure parcel system and the encumbrance rules were simplified to the point of absurdity (you mean my strength 8 wizard can pack 80 pounds of gear in his backpack without even slowing down? We should ALL be so fit). This is what I find so encouraging about the idea of stripping everything down to being a function of the ability scores and then tweaking that foundation. If they can strip the core game down to a single, simple mechanic then add the complexity in through modular components like themes and kits, and subsystems like feats; then I think it is very possible for them to recapture the classic feel with a more unified, or "modern," design approach.
Personally, it is my biggest hope that they give the next incarnation more support for homebrew elements than the 3 obscure pages devoted to it in the 4E core materials.
Edit: Minor correction in 2nd paragraph.
