PL Theory makes you start looking at everything like a PL problem. APIs become little languages, and rich APIs become larger ones. Parsing becomes trivial, lexing even moreso, and everything sort of devolves into thinking about interpreter state. Then it gets frustrating because you can't just pick up the API interactions and rewrite them more-efficiently, and you end up chasing your own tail wishing you could.

I think it's made me a better programmer, but it must be tempered by understanding the limitations of how and when languages are worth building.