I'll usually put something like 'feel free to crit' on anything I post, but if their "crit" is just them sniping I would go off. "You should focus on ABC to bring out fill in the blank", that's fine and appreciated because I know I'm far from being the top of my game. But just something like "you suck" and no backup or explanation; then I'd do my best to put them in check.
Criticism is always going to happen whenever we offer our work up to the public, the more your in the spotlight the more you're going to get people with something to say. It seems most crits given are crits given just for the sake of giving negative criticism. If they're not negative they're just a generic "Wow I love it" or something along those lines.
Of course it's always a boost to the ego to hear "Wow that's great"; but I, personally would like people to take the time to type just a little more. What specifically do you like about a piece? With popular artists it tends to be a lot of the same. I'm not popular so I wouldn't know what it's like exactly, but from an outside perspective, it seems like "Good Job" doesn't hold much weight, especially when you get it a lot. But something like "I really like this piece because of how you captured this part blah blah etc." would really make you feel prouder of your work and would make you appreciate that comment (and commenter) even more for taking the time to really look at your piece and think about it, and get a real impression of it, before posting. Every artist has specific things they like about a piece, even if they don't particularly like the entire piece, it's always gratifying if someone else sees what you see and appreciate that little part as much as you do. (It's funny but I've known a few artists to be most proud of something as simple as a line and how it effects the piece.)
Keeping that in mind, a negative comment, unsolicited or requested, tend to just be what a lot of people above described it as. Complaints, "this sucks", clueless 'know-it-alls', etc. If a crit is a real crit with a goal of helping (always keeping in mind the style of a piece) then I welcome it. It's one thing to say "His pose is wrong" with a total disregard to the stylization, specifics, and suggestions for improvement; it's a waste of time for someone to write. But if after looking at a piece and you notice something specific like "With that angle and style it seems like the right hand is a little too large" then IF you have the knowledge offer a suggestion on how to improve this piece, or help you avoid the same mistake in the future. It's things like that, that I feel, make a crit a real crit. Things like that make a comment worth reading, when you know someone took the time to think about what they had to say and why they wanted to say it. And a comment like that is always welcome to me.
I'm torn between 3 and 4, but I ended up picking 4. I don't mind a little constructive critiques here and there, but only if I ask for them or am expecting them. I hate when people just barge onto my page and start telling me what's wrong with my art without even considering style.
Bottom line for me...I think taking advice and critiques to heart hinders an artist. You are stripped of your own originality to fit into someone elses idea of what art and style are suppoused to be. If you can't just draw things that you figure out yourself(ofcourse you can use refs, but not styles) then I think they're pretty useless artists that won't really get anywhere unless they're using someone elses methods.
If all of the comments are completely complementary I get the feeling they're all lies. The same goes for someone who only says nice things. If someone consistently sounds like they know what they're talking about and is respectful, they're going to be my favorite commenter. (I don't think I've ever really gotten any disrespectful crit, I know some cool kids.)
I think the phrasing of your last option might have discouraged some folks from honestly selecting it.
Having said that, I think if you're not willing to entertain and talk about people's criticisms, then you're shooting yourself in the foot, because you're pushing away opportunities to learn and grow as an artist.