It's practically impossible Ruben. On every site you "need" to rank users based on something. If not, there's no way of ranking content. You would get frustrated on a site that has no ranking systems. Even on Dribbble, you can see a lot of "crappy" work that probably shouldn't be there. There are lot's of guys that don't put to much effort on making "content"pretty and you certainly don't want to look that stuff on a site that you most probably use for inspiration. No matter the system (because let's face it, not even the Dribbble invite system is bulletproof) you will get guys that post shitty work and that doesn't meet standards. And maybe I'm one of those guys. That's why likes and followers are necessary, and it's not all just about vanity. The shots stream you have on Dribbble is huge, and nobody pays you to review them. I don't think you'd enjoy a place where there's no "quality meter" and almost all of the work you see is below standard.

Regarding the Facebook's M comments... Their M logo doesn't "vaguely look like another logo". It's almost identical. So THAT kind of comments are constructive. They are aggressive, yes. They are surely not the most friendly way of telling someone they've made a mistake. But that's probably something you can expect from designers, since nobody likes to be copied.

Kind of off-topic PS: You also need to consider we are not all experts on every piece of design "type" that gets posted on Dribbble. Check out my comment here. I really liked the shot, specially it's colors. But I know very little about illustration, so what kind of comment do you expect from me? An "I really like this!" comment is vague and imprecise, but I cannot give an illustrator quality feedback without being an illustrator myself (considering the illustration doesn't suck), but I'm still allowed to leave a cheerful comment recognizing he did a nice job.