A good RPG system is useful. I can state, with fair certainty, that a useless RPG system is not good.*

What makes a RPG system useful largely depends on who is using it, and what they are using it for. I want a system which I can run easily, which can interpret player actions into mechanics fairly well, and which I can teach other people if I need to. Fate does this. So, Fate is a good system for me. I also want systems which give a fairly distinct setting and playstyle, without being overly obtuse. Shadowrun is a fairly good example of this. So Shadowrun is a good system for me as well, even if it's one I use much less frequently.

I would say that a good RPG system (in general) has a goal and is successful at presenting and accomplishing that goal. It doesn't need to be a system for me. It doesn't really need to be a system for anyone, technically. As long as a system has a specific goal in mind, and chooses rules and mechanics which accomplish that goal well, then it can be a good system - at least, at that specific goal. No system can reasonably be good at everything, and even the generic RPGs are generally not as good at specific settings or scenarios than some RPGs specific for those settings.

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* A system which highlights how useless it is to display useless RPG mechanics would technically be a use of the system, and thus potentially make it good at displaying bad RPG mechanics. So you can put your Time Cube RPG printouts back on the shelf.