I've been thinking about the essence of D&D comment.
Extrapolating from the Legend & Lore columns I believe the design team are planning on keeping some very central statistics of D&D throughout every edition and building them into the core game, something slimmed down yet essentially recognizable as D&D.
What does that account of? Likely levels, classes, and races along with the 6 ability scores. I also think they will probably build into it HP, AC, Saving Throws, move speeds, and a gold piece coinage system which can be used to buy equipment, arms, and armor (to begin with). Other elements may follow in supplements, or could even be built into the core, but the "essentials" comment may refer to the aforementioned.
What made me think of this was how 4e uses levels to refer to challenges and monsters. It is possible for these to be kept for a simple monster construction system for a 3.x compatible supplement. d20 Challenge Ratings can be somewhat overlaid into a level system. AD&D's monster rating by XP could also be converted into such a Level-based system. In fact, I think it may already have been done in one of the old Dragon magazines. For both pre-4e system the fraction or spread within a level, especially first, will probably need to be worked out, but I don't see why it couldn't work as an option. For instance, a 3.x kobold is 1/6 of a level 1 monster and 1/2 of one for AD&D (though I suspect they'll drop XP by HP and random HP generation for it. Lots of people don't know why it was ever there to begin with).