My advice is basically to not be a dick. I think I'd like feedback if it wasn't dickish, even from other players and not just the GM. I know I have comma problems and feel like it'd be nice to have someone just tell me what is up from a readability standpoint. I think the worry with these sorts of conversations is that certain personality types relish the opportunity to make someone feel inferior, and that's a bad dynamic. 

All the same, feedback is good. It's just that feedback cannot cross the line to where it's some sort of mechanism where the critic feels validated by the pedagogy. The person offering criticism should need/want to see the other person improve and see worth. If it's just some dude getting off on how awesome he is by handing down lessons to others, playing that sensei role from mom's basement, it's a shit dynamic.

I have no idea how to offer that criticism. It's not an easy conversation to have. I know when I've told someone that they are not up to the standard for an RP, I've felt all kinds of crappy for saying it out loud, but felt obligated to on behalf of my other players.

tl;dr: Critics should be humble as fuck.