When I review, I try to make a point of saying at least one good thing the author has done right and at least one bad thing the author could improve on. Once that's done, I write my general impressions and let my judgment tell me if I need to add more praise or point out more stuff that could be fixed. Above all, I try to be fair and polite, because I've come across as a troll/flamer before, since I have a hard time "cushioning" criticism. If I think a character is a dick, I'll tell you he's a dick. If I think that the sex scene lacked realism, that's what you'll be hearing from me. But lately I've been trying to sound more humane. It's a selfish thing, really, because if the author is offended by my review, they'll disregard it, making me waste the time I spent writing the review in the first place.
When I receive reviews, I am oh so very wary of praise. It's not that I don't like it (on the contrary!), I just don't fully believe it. When I see someone saying "Oh, you're such a great writer!" or "This story is awesome!" I just narrow my eyes and go "...okay. Thank you!" and wonder if they really meant it. Of course, longer reviews like the kind I write are actually heavenly, because I can get a better idea of what the reader actually thinks. Those are the reviews that make me keep on writing.
But on a broader scope, I think that you can't improve without criticism. A single review that points out your flaws is much, much better than 100 reviews that praise how awesome you are. If you can have both, great, if not, tough shit, life ain't perfect. I do think that people should give more feedback, of ANY kind, but that's probably what 99% of all authors around here believe.