Nothing. Been using arch (and ubuntu previously) exclusively for years. Recently finished darksouls 1 and 2. Been playing elden ring this week. The vast majority of games I want to play either just work or require a one time config change.
There are tons of reports on Proton DB (and my experience agrees) where there are slow downs and stutters that just don't happen on Windows. Spyro: Reignited and Batman Arkham City are some recent examples for me.
I've played a few games that I can run at 4K with frame rates I don't notice any problem in, where with Proton it isn't the same experience.