Use this, just in case you're unsure of how to really really comment on the artwork of others.

#1 - Never give critique unless there's an enormous button emphasizing the artist's request for it. Otherwise you should probably just slaver over it and bang out a bunch of meaningless tripe about how good it is, or refrain from saying anything at all.

#2 - If you see someone else disagreeing with your opinion over a piece of artwork, you should immediately run to their gallery and pick out some of their less-appealing work, then run back and use that piece specifically to degrade and demoralize them. That way you can assert your own intelligence and totally not look like a huge idiot.

ie, "If I were you, I would take a look at my own gallery before scrutinizing that of another's. I mean, after all, taking snapshots of your cat is hardly notable. (insert generally well-meaning but now-sarcastic emoticon here to drive your point home)."

#3 - Remember that the internet is serious business and that DeviantArt is only for super talented folks that have an education, not a giant melting pot of all levels and types of artistry. If you see someone who isn't slapped with at least a four-year degree in the area of art on which they are commenting, make sure to heckle them and then refer to Rule #2.

ie, "Your statement is unwarranted and insignificant because I have had ten years of experience with photography and am currently earning my master's degree while studying abroad in Nicaragua."

#4 - If a piece is featured as a Daily Deviation - which, in the grand scheme of things, is a truly enormous reward that you can splatter across your resume - don't even bother to think that it isn't appealing to you, and certainly don't voice it. (For Pete's sake, DD's are chosen because everyone likes them. And if they don't, then refer to Rule #3 because they're probably uneducated trolls.) Remember that fragile egos are lingering behind the thumbnail and that providing any sort of honest feedback will not only crush that ego, but will also fail to give the artist any true insight or room to improve. Remember, only pats on the back and hearty congratulations make a person well-rounded.

#5 - If you like anything that is conventional or generally appealing, you know nothing about art so don't even bother. The only time you're a real artist is if you comment on and give praise to crap that only "real artists" have insight to.