As with most things in life. If you haven't experienced, tasted, seen, fealt, heard, etc.. whatever it is your critiqing, your opinion is shit.
I agree, and drive-by "excellent work" comments are meaningless, as far as I'm concerned, especially if someone hasn't even USED/evaluated the work in question. If that's all you're doing, then you might as well be saying "great preview" because that's all you're really commenting on. I guess it's just another trend, we have Vista-trendwhoring, now comment whoring?
I rarely leave comments on skins, and when I do, I make sure I *really* have something to say other than the blind "great work" because I know it will mean more when there's actual content and substance to the comment, and I'm damn sure a skinner will appreciate that more than someone just taking a quick glance and commenting on what turns out to be just the preview, and not the actual work itself.