I feel any thread which becomes a list of things people won't actually care about, for example what the weather in some random place in the world, and creates no conversation, is just a spam thread.
If the thread is a list but of something appreciated (a good photo or video thread, for example), or if people actually create conversation and don't just add their bit and never post again, then it's fine by me.
As TVE points out, any thread sufficiently derailed, or turning unpleasant, is fair target for the padlock too.