Surely you've been in the situation where something popular or acclaimed annoys or angers you and you have difficulty figuring out why. This seems like a situation where the various layers of annoyance are thick enough that it's worth discussing the comics on a case by case basis. Also, sometimes the author of the blog points out successful jokes on Munroe's part. It's not nearly as mean-spirited as it could be. That said, I also disagree with the premise that criticism, even public, overwhelmingly negative criticism is somehow sociopathic, and with the premise that a critical project is not only subservient to "real creativity" but is also useless. Of course, some criticism--like the usenet trolls Cadre discusses--is shitty. But lots of art is as well. And there's plenty of room for creativity in criticism--"critical theorists" like Walter Benjamin and Michel Foucault are considered by many to be among the best writers of the 20th century. You say that the energy spent on xkcd sucks would be better spent on "creating real stuff," but even more than "real stuff" (yay, another forgettable, mediocre webcomic) I'd like to see a blog intelligently and cogently drawing attention to great webcomics. I mean, consider how XKCD Sucks compares to, say, Adam Cadre's reviews. Did you see the hatchet job he did on American Beauty? Or, uh, Lanark? Not like I think these things deserve plaudits--far from it. But isn't it satisfying to hear someone smart explain why that shit is whack?
Sound points, though I suspect a few were born of semantic confusion. We should put some distance between criticism — which is not a term I used in the original post — and complaint. Obviously, someone who posts as much writing about cultural works as myself wouldn't express hostility toward the critical enterprise, let alone operate from the premise that it's useless. And I think I've discussed approvingly enough critics of all different stripes here — Wesley Morris, Robert Christgau, Roger Ebert, James Wood, Manohla Dargis and Anthony Lane all come to mind — that a charge of disapproval of criticism itself can't stick.
When I think of what critics do, I think essentially of in-depth description. "Movie describer" is a common slag on film critics, but what else should they do? (Some critics fall all over themselves to put the works they describe in grand sociopolitical and/or economic contexts, usually without getting past the falling-over stage.) One can describe a picture, or any work, well or poorly. Their descriptions can take a positive or negative tone, or at best a bit of both. But a critic's chief task seems to me to be translation from one medium to another.
Complaint differs in that it begins from, rather than arrives at, a negative position such as, well, "xkcd sucks." Like a story with a contrived ending or an essay that pretends to arrive spontaneously at its prescribed conclusion, this is way duller than it could be. "Hating" a work or saying a work "sucks" could hardly be more common, but I submit that it could also hardly be less justified. (A work can't even take action; how on Earth could it earn hatred?) When I hear someone rain opprobrium down on a piece of literature, music or cinema, I almost never trust their judgment. The extremity suggests not only a lack of imagination but a lack of thought. A decent critical essay presents a coherent analysis; "this sucks," no matter how many words a writer takes to spell it out, barely exceeds the level of impulse.
I find the good/bad divide less useful with time and the interesting/uninteresting more so, not least because why a work is interesting or uninteresting makes for a richer essay than writing why a work is good or bad. We can't have too many people pointing out what's interesting in the world, which is why I agree with the reader's comment that "a blog intelligently and cogently drawing attention to great webcomics" would be a fine thing indeed. But note that, when I urge people to "make something better," a "forgettable, mediocre webcomic" does not count as something better.
As for critical theorists like Foucault residing "among the best writers of the 20th century," we must return to where we started: