With an interesting setting coupled with Monte Cooks excellent layout decisions. MCG set the ground work that would give birth the Cypher System. It is hard to argue that Numenera is not an excellently written RPG. In Ptolus we see the perfection of the layout and writing style of Monte Cook. In Numenera we see the evolved rule system that Monte Cook had been considering and I think may have even suggested from 5E D&D before he left the creative team.
Numenera is fresh take on using a D20 at the tabe for everything, and a whole new take on character creation. This was refreshing and eagerly embraced by the fan base. Cyphers themselves were by design constantly rewarding the players with items that they could use to influence the game. Polishing the whole experience off by telling the GM to just focus on the game and not worry about rolling dice. Over all at least in my opinion it was a wild success at writing an RPG.