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.
As the rules stand now, players in the Kill It With Fire RPG don't start as "regular guys" because they get 1 HD for each body point they have, as well as their level in hit dice, but DMs looking for a more risky starting game could certainly have players start at 1 HD.
As for monsters, the hit die allocating will mostly represent flavor; cannon fodder like goblins or normal people will usually have one measly hit die. NPCs don't follow the same rules as PCs. They are just there to be fun challenges.