"It's generally accepted in fandom that if you can't say something nice, don't say anything at all."

Well, there's some truth to that. Say if I don't like S/J, I'm not going to go to a S/J forum or post in a S/J fan's journal about how awful S/J is for both characters. Whatever stirs your coffee, ya know? I have seen anti-slash folks go into slash-friendly forums just to expound upon the evils of slash though.

But my respect for "private spaces" doesn't mean that I won't keep quiet about it in general discussion of the show. I'm just not a believer in "cheerleader" forums. No show is perfect, and if you can only say nice things, there's only so much you can talk about. The key is to discuss the show -- the writing, the acting, etc. -- and not the fan personalities involved in the discussion.