The problem with the Twilight example is that there really often is an undertone of "...and if you like it, you're an idiot." I see it much more in other places than I do here on ehell, and there are posters (Cathy, for example) who could say that they think Twilight sucks, and I wouldn't think anything of it. But there really have been times, even here, where I've sensed that undertone of "Twilight fans are idiots with no taste." It really is specific to Twilight (and I'm sure a few other books/movies/etc), if someone said to me "Stephen King is garbage," or "rock music is garbage," or "dogs suck," or "potatoes are disgusting," it wouldn't occur to me at all that I was being judged for liking rock music and dogs and potatoes and Stephen King. It's different for Twilight, because frankly, I am often judged for liking it, and there is very much a "Twilight fan" stereotype that many of us don't appreciate being wedged into.
Cathy does have a point about diplomacy when expressing distaste for something, though. You don't necessarily need to say "In my opinion and only my opinion, and this is only how I feel, I think Twilight isn't the best book in the universe," but I think it's much better to say "I didn't like Twilight at all," or "I don't think Twilight is appropriate for kids under 18" or whatever, rather than just declaring it garbage.
As for the original question, I've been frustrated by that behavior, but I'm probably guilty of it, as well. I do think it's a good idea, if you find you've made 50% of the posts in a thread, to look back at what you're writing and ask if you're actually adding anything new.
(there were five replies while I was posting, so if I seem behind the times here, I'll post again)