I, too, feel that the OP is treading-on-thin in regards to the "edition wars" rule by admin. However, I think the general question is a good one; what defines D&D for you?
For the sake of full disclosure, I started role playing with a basic edition box set, went to 2nd edition, really loved 3rd edition, and am currently interested in Pathfinder. While I understand the appeal of 4th edition, it just never appealed to me personally.
My "core" idea of what D&D is includes Humans, Elves, Dwarves, Halflings, Gnomes, and probably Orcs as character races. The four core classes, in my opinion, are Fighter, Rogue, Wizard, Cleric. If we could expand a little bit on those, I would throw in Bard, Druid, Ranger, and Paladin. Six ability scores: Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma. The core D&D experience is the dungeon crawl, culminating in a final showdown with a dragon. D20s are used for most roles, although other dice are used for damage. Hit Points, saving throws (a simple ability score check is fine), Armor Class, Vancian Casting, and some manner of Multiclassing.
NOW comes the more important question of details. I don't want to return to 1st or 2nd edition multiclassing; I prefer 3rd edition multiclassing, but I realize it had a lot of problems. I don't *love* any of the skill systems in any of the versions of D&D; I prefer 3rd edition skills, but, again, I realize that it has several problems and needs to be reconsidered. I don't want to return to races as classes. I don't want to return to level caps for non-human races. I don't want Vancian Casting as it was done in any of the previous editions. I don't want my character to be fine and then just die at 0 hit points. I don't like 4th edition's jargon and meta-gaminess or 3rd edition's complicated rules. I don't like too many magic weapons, as seen in 3rd and 4th edition. I don't want save vs. death or level drain. I don't want to feel overwhelmed with all the choices I could make, but I still want flexability.
So what do I want? I want a more flexable version of Vancian Casting, which is more akin to Spontaneous casting of 3rd edition. Warlock/4th edition type casting is good too, but not as the central type of magic. I want a hybrid of 4th and 3rd edition multiclassing... although I'm honestly not sure how that would work. I want skills that are easy to use and highly customizable. I want easy character creation - whether it's a 1st level character or a 20th level character. I want feats... I want magic items to be special again. I want the base rules that focus on "theater of the mind" type play, rather than miniatures and battlemats; those can be added on if wanted.
It's a bloody tall order. I have *some* ideas of how 5th edition might be able to pull it off, but there are some problems I can't quite wrap my brain around. Hopefully, the pro's know what they are doing; otherwise, I might just write my own version of D&D.