GURPS (and to a lesser extent EABA) taught me how good consistent logarithmic scaling is, the concept of free lunches and package deals and what makes them iffy in system design. And the 'it costs what it costs for the utility, regardless of flavour' principle.
Transhuman Space taught me to differentiate between continuity- and pattern-based theory of the self.
Transhuman Space along with Eclipse Phase taught me to reconsider how a body-hopping-OK system should be designed (though I don't agree with all the design decisions).
Exalted, WoD and FATE Core taught me to appreciate the 20-30-Skill minimalism (i.e. when everything is covered by them, no Custom Skills required).
Exalted also taught me linear Dawns, quadratic Sorcerers.
Golden Sky Stories and Active Exploits gave me an idea what diceless systems with resources should look like (even though I consider the specific implementation very flawed).
Cortex seems to have so far only showed me that I don't enjoy irreducible swinginess, even more so than I thought.
Mindjammer taught me the horrors of the stovepipe design antipattern applied to RPGs.
Theatrix may not have taught me anything specific, but contributed indirectly to my vision of what a diceless or low-randomness but high-characterisation RPG should look like.