A NES with satisfied players is a succesful NES. A NES with very happy players is a brilliant NES. The. End. There is no reason to measure it. Regarding BJs post, I'm supporting him, as most players are satisfied when a) the NES is entertaining mixed with b) the NES lasts a little long. Note, they are satisfied, they enjoy it.
I think there's a confusion of causality here. A good NES makes happy players. We can all agree on that. However, to say that happy players "make" a good NES is meaningless. Happy players might
imply a good NES, but it's the NES that makes the players happy, not the other way around.
However, when you're trying to figure out how to make a good NES, the advice "make your players happy" isn't all that useful. It's a good ultimate goal, but it leaves the intermediate steps up to the mod. Furthermore, the four styles of NESing are preferred by different segments of the population, so you can't make all of your potential players happy with one NES.
So the solution, in my opinion, is to have an unbiased, subjective rating system that spells out the
qualities in each particular NES. Simulationists can restrict themselves to the super-high realism ones, while Arcaders can join freer, more abstract games. Storyists can join highly complex game worlds, while Boardgamers can opt for pure tactical action. Rating NESes as universally "good" or "bad" is obviously futile, but we can say that a NES is realistic, or flexible, or novel in its mechanics without getting all dictatorial.
These are the things that can't be measured by raw stats alone, and they work as a tool for both players and mods. A player who wants fast updates and loose rules can find such a game without having to do the research himself, and a mod who wants to make the deepest, most epic NES possible can check an archive for inspiration from old NESes that are similar to what he's aiming for. It can be fair, it can be informative, and it can be useful, and I really don't know what else you could ask for. At the very least, it's worth a try.