Well, D&D-ness is an illusion. But that's the case with most words. The meanings we attach to them differ and often are greater than a mere unique descriptor. De Jure WotC (or whatever will replace them) can attach the name "D&D" to anything they want. D&D 4E is a variant of checkers with spiked and poisoned pieces? Great, at least it's not a CCG!
But a lot of people will disagree. There's a certain quality attached to the word. Not many people will agree on the exact definition, but it goes beyond a mere trademark. Such is the way with legacies. Sometimes a product can survive major changes, sometimes it can't. The new Mini Coopers retained something of their quality, most movie sequels didn't.
So, what is "D&D-ness" and what would you have to do to the game to change that? Characters with different abilities form a group, overcome obstacles and gain improvement from the rewards they reap. This is the core of the D&D experience, I think it would survive without the window dressings of levels and classes, though those are pretty neat ways to achieve the gameplay mentioned before.
Another aspect of "D&D-ness" would be some elements it has gathered during its long history. I don't think they're really neccesary, but they form a certain frame to enhance the legacy. Dungeons, dragons, magic missiles, demons, orcs, kobolds, named spells, tarrasques, beholders...