I write my stories mainly to entertain people, but I believe that I can't do so terribly well if I'm not out to improve my writing. This doesn't just come from negative reviewing and constructive criticism, but also from thoughtful praise and discussion. One of my favorite comments was not an especially long one, on my story "Invisible" at SQ--someone pointed out, very casually, that the story was loaded with doors, and Snape goes through every single one of them. That's my favorite type of review, because it points out something in my story that I hadn't seen before, but when I look now, I can't not see it. And that improved my writing, because now after I read through a story, I'll look at it and see what's just screaming out of the text at me, and if it needs polishing to shine more prettily, I'll polish it. The same thing happened in a review of a SW fic, where someone mentioned how my writing partners and I were really focusing on using color symbolism--we hadn't done it deliberately, but we'd all been doing it, and after that, we used color in a more concerted--and effective--way.

As to people leaving one-liner reviews, I don't object to it, exactly, but of course it's not as great. Still, on sites that don't let an author keep a hit count on her stories (fewer and fewer seem to be doing this), it's a way to see how many people are reading.

I don't, however, think that a comment board is a place for beta-ing a story. A beta is a more private process, where the authors worst gaffes should be fixed before the story is exposed to the public. A gentle, "Did you notice that..." is one thing, but going line by line and making corrections is a bit much to actually attach to a story in its finished and public form.