As the title suggests, I'm trying to get at a fairly simple, although potentially hard to define, concept: what makes System A better than System B?
An interesting thing to try, but I suspect your effort is doomed to failure. Not only are there many variables, but there are also many ways to interpret them. Different things are important to different people, so you are going to get conflicting answers. Just take a look at some of the threads on this board; debates about whether some aspect of the game is more or less important than other aspects get many responses.
Some people want a mechanically balanced system, some prefer to have a more immersive game even if certain elements are imperfectly balanced. Some people want a greater degree of simulation in the rule set, others just want the game to be quick to play. Some people enjoy classed based systems, others prefer point based games where you can purchase any ability within a set budget. Some people like having clearly defined rules that spell out what a player can and cannot do, others prefer to leave things up to the DM. And then of course you have the impact of what setting the basic rules assume; Some people like Shadowrun more than D&D because the like sci-fi more than fantasy, others because the prefer the rules.
The various versions of D&D fall pretty close to the median preference of the average gamer, though 4th edition is different enough from the previous editions that it is not quite in the same area as earlier editions. But while many prefer some version of D&D to the other systems available, there are nearly as many who like that system, but like a different system more.
It is like the classic definition of pornography. You might not be able to directly define what makes a given game system great, but you will know it when you see it.
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