I like how you assume that somehow I'm attacking the show when I'm the only mod who actually watches and enjoys it, which is why I'm posting to give the heads up that it is going to be moved.
We generally move eternally fast-moving threads which have built a sort of sub-community. Basically when most of the posts are no longer talking about the actual content of the show, but rather community stuff instead which includes but is not limited to: fan art, fan fiction, general subculture of the show, general interactions between members, etc. Most of the time, this applies to popular games. MLP is definitely notable because the thread continues to frequently the top of the OT forum, regardless of actual discussion about a given episode.
The building of the community forum is something relatively recent. If such a practice was around when Lost was airing, it would almost certainly have also gone into the community forum after it finished airing.