With all the speculation floating around about what 5E could possibly look like, or what folks would like it to look like, its interesting to ponder what exactly makes D&D work in the first place. The simple fact that people are out there still playing the earliest iteration of D&D means that there are some definite "core" elements that keep this game alive.
One of the first things that springs to mind is the Hit Die. This is, I think, the most important baseline of the game, and what everything else is built off of. Starting with OD&D, the baseline is this: a creature has 1 HD, or 1d6 hit points. An attack against it with, say, a sword, does 1d6 points of damage.
This is deceptively simple, but its an important reason why D&D works. Starting with this baseline, a regular sword has a chance to kill a regular guy in 1 round. The randomness of tossing dice to determine the regular guys hit points, whether or not you hit the regular guy, and how much damage you do if you do hit the regular guy, means nothing is predetermined, nothing can be taken for granted, and there will always be an element of risk.
As a baseline, this means you have something to measure against when someone gets better than the average guy (or "levels up"). More hit points, a better chance of hitting, etc. Or maybe your magic sword gets better than the regular sword. It lets you quantify tough concepts in a simple way that doesn't take hours to figure out, or bog down your game resolving simple combats so you can get on to the rest of the session.
For old school gamers, this can be taken for granted. Subsequent editions have gradually eroded away this essential baseline. 3E emphasized things like ability modifiers for monsters. A sword still did 1d8 points of damage against an orc, but that orc was now likely to have a Con modifier to its hit points and do a whole lot of damage, often more than that 1st level character could withstand. This created the expectation that in order for one's 1st level character to survive and succeed, its player would need to do some serious min/maxing, arranging scores and abilities for the optimum number of high modifiers to boost damage and hit points. Characters became a lot less randomly generated, and lot more "built", carefully constructed to counter an increasingly spiky baseline.
4E further eroded the baseline. Some might argue it practically washed it away. Orcs and PCs alike start out with 20+ hit points. Yet that sword is still doing 1d8 points of damage. The simplest element that made D&D work was removed: in one attack, a sword has a chance to kill you. No longer. In this latest edition, in addition to min-maxing a character, now groups had to min-max adventuring parties! It was necessary to have the best combination of striker, defender, controller, etc, so that those four or five PCs could carefully coordinate their attacks on the game board to kill that one orc.
The farther you get away from the 1 Hit Die baseline, the farther you get from quickly resolved combats, abstract resolutions that lend themselves to imaginative interpretations and descriptions, and sessions that emphasize goals and explorations over the minutiae of combat.
We'll look at another element of what makes D&D work tomorrow.