The most frustrating thing about this game as a PC player is the regressions in functionality from the series. It feels like improvements aren't merged back upstream and then those improvements aren't in new releases.
Stuff like an in-game framerate counter, not being able to turn in the audio down (or off) in game, and (maybe?) the better multicore support. But I'm getting very good and consistent performance when I bring my refresh rate down to 60Hz and disable G-Sync.
FWIW, G-Sync worked in the demo but I can't seem to get it to work in the full release, so that can be considered a regression too.
During the early days of Forza on PC talk, we heard a lot about the team getting the tech ready for PC and how they were preparing for it, and whilst it does seem fundamentally strong, the recurring issues that had been fixed in previous releases indicate an odd issue in the development processes.
(FWIW, I am *really* enjoying the game - Turn10 are overall doing a fantastic job, just a few frustrating points).