I can only second Euryleia there. I'd never object to a "nicely done", but it's people pointing out mistakes and ways to put something better who help improve my writing, no matter if it's a bdsm fic or a classic short story. This needn't always be a complete analysis of what went wrong. If I'd get three reviews in a row telling me "there's something off in the middle of the story, but I can't really name it", I might be able to spot it myself.