Dale Peck, he of Hatchet Jobs fame, once said, "Literature does have its enemies, and chief among them are pseudointellectual artists and critics who think their love of books translates into some kind of knowledge."
This problem is writ large in SF&F fandom, where most of the criticism has passed into the hands of fans. What sounds like a good idea on the face of it — fans serving other fans! what's the problem? — has a downside: fans can be both the best and worst evangelists for, or interpreters of, something.
From what I can tell, the only serious and informed criticism of SF&F is taking place in venues that the average reader of SF&F never runs into, and has no inclination to touch in the first place. There are some remarkably informed and thoughtful scholarly and literary studies of SF&F out there, not one word of which has the slightest impact on the reader and his single biggest question: What's worth reading and why?
Criticism-as-a-trade has passed largely into the hands of inspired amateurs in big part because there's no money to be made in such a trade, and no careers to be made except in the most rarefied ways. The sheer volume of published SF&F, of published fiction period, all but guarantees the professional critics are going to have their hands full with just the A-list authors. When SF&F does get considered in the pages of, say, the New York Times Book Review, it's inevitably an A-list fiction writer straying into that territory (Colson Whitehead's Zone One, or Justin Cronin's The Passage). It's almost never SF&F-as-such, in big part because that market has such churn, and because it's allowed itself to become immune to criticism in the worst sense of that term.
Any talk of what's worth reading has become the province of folks who attempt to make up for nuance with enthusiasm. They are not ill-read; it's through many of them that I've received recommendations for some very good books. But they often seem uninterested in what would make one book better than another outside of the circle of their prejudices. (I am not claiming I know any better about such things, only that I at least know my prejudices are at work and often trip me up badly.) The end result is hyperbole, bandwagoneering, and the if-you-liked-that-you'll-like-this variety of criticism that makes me reach for back issues of Kirkus as a palate-cleanser. There is no sense that criticism can work as a way to single out positive and negative examples for both readers and authors.
For contrast, I see far more nuanced criticism of filmed SF&F out there amongst film fans than I see criticism of literary SF&F. A big part of why is because the movie buffs on the Web have better models to draw on: Roger Ebert and Harvey Karten come to mind in the old school; maybe Drew McWeeny as a more recent example. Literary SF&F has few such broadly-recognized champions, in big part because the job is so thankless and the stakes so small — and because, as Peck pointed out, it is easy, too easy, to have your mere love of something be interpreted by others as wisdom or insight.
Just because I adore something does not mean I have a privileged understanding of it. If anything, my understanding of it may be all the muddier because I am unwilling to ignore my own prejudices. This is why the idea of "loving something despite its flaws" seems backwards, or at least incomplete. The fact that we look for flaws in something can be the manifestation of a form of affection for what it represents as a whole. We love the thing in question enough to want to know how to see beyond it towards something better. (This is why I no longer enjoy the likes of RiffTrax or MST3K: why celebrate movies at their most pathetic and incompetent when there are so many genuinely good ones that remain unseen, even by knowledgeable fans?)
The more we love SF&F "despite its flaws" without learning from them, the more we find reasons to be forgiving of things that downgrade the field as a whole. Our being on guard against the worst excesses is not out of a lack of love and never has been. If we love SF, then we need to do all we can to bring better incarnations of it into existence, and that means singling out its failings without undeserved rancor. As Nick Lowe would have it, yes, you do have to be cruel to be kind, in the right measure.
What I'd like to see, then, are more critics of literary SF that tackle the subject on two fronts. On the one hand, they should be able to tell us whether or not the book is worth reading and in what light: a reader's review. On the other hand, they should also be able to speak about the book's real literary value — not just whether or not it's "well-written" but whether or not the author understands the true implications of what he's bringing up, whether other books in the same vein would constitute an advance of the genre or a contraction of it, and so on. We should see more of these things together in the same package, informing each other, and too often we have to go to completely different groups of people to get it. The more examples we have to draw on and emulate, even if not from our own field, the better.