I rarely comment on mixes because I don't know enough to help.
See, I think that's a mistake. I can't speak for anyone but myself, but I appreciate getting any kind of feedback from anyone. The more the merrier. I could care less what my perception of the listener's level of experience is, I just like that they were interested enough to comment. It's up to the poster to sift through and decide what feedback works for them or not. Even just high-level commentary that doesn't get into finer details or technical jargon can be nice and/or helpful.
Contrary to that though, you do sometimes see someone give perfectly reasonable feedback like, " I'm not 100% sure, but it seems to me like you could bring up the bass quite a bit..." or whatever, and then the OP is like, "What is your listening chain?" "I'm using (insert expensive monitor brand name here) in a well treated room and it sound fine to me". That kind of stuff drives me nuts, and I think discourages someone from jumping in if they don't already feel "qualified".
While I've been here a long time, I never really developed a deep understanding of the technical aspects of things and still get intimidated about giving anyone else feedback or advice. That doesn't stop me though!
I hope those long time members (and frankly those who have the most information to offer) are looking at more than just the mix section to decide who they will listen to. Some of us may not have much to offer in the way of mixing help but we try to be active in other parts of the forum where we can add something to the conversation.
Again, just speaking for myself, I don't put that much thought into it. You make a good point in that someone can certainly be active and helpful on this site, but not so much in this specific forum.
Personally, I tend to comment when;
1) The poster is someone I already know/like - regardless of the music itself
2) I really like the music - regardless of who posted it
3) I hear something I actually think I can help them with - regardless of the music itself or who posted it.