I would really like to see the [SPOILER][/SPOILER] tags, preferably renamed as Hide tags. This can be really useful in order to hide Quotes. They are usually reminders that you add for your post to make sense, but most of the times people have already read the post quoted. Or think of big chunks of code. Often you tell somebody "post some code" and they post their whole program. It would be preferable this to be hidden by default and expanded if needed. But even in the simple occasion that you want to hide something because you don't see it as significant or it is just additional information but you don't want your main point to be missed.
Give me your comments.