I don't think mods should ever change the content of any user's post or comment. Delete? Sure. Edit? No.

With that said, I don't see changing the form or metadata as changing the content, so tags or NSFW are fair game from my point of view.

I also think that the ability to merge posts, as long as they are in the same community, could be useful. Think of a scenario where several posts are made in a short timeframe on the same topic. This splits the discussion and is kinda spamming. Merging would keep things clean without losing data. Something like moving the duplicates as comments under one post and all comments from them as replies. But if this involves a delete and rewrite as opposed to a trackable reference point change, than it can get iffy, as it counts as posting something in someone's name. I don't really know how things are implemented so I don't know if this is possible, just something to think about.