Vampire: Bloodlines had this system as well, by the way.
I think the system is fine, and it reminds me very much of my pen and paper days. I don't think we'd even dream of rewarding our players from running around killing everything in sight, but at the end of each session you'd typically get some XP, even if you hadn't fought anything.
Yeah, I'm sure this isn't new to PoE, just that it's pretty rare in the genre in general these days, and the game doesn't explain this as well as perhaps it could.
I actually like this XP method, as it's more about what my character does in the world at large, and less about how many kobolds he skewers (a lot).