Oh, god, there's a lot that could wreck up PC for me (and everyone has already beaten me to saying the features but I'm going to write about them anyway).
The worst case scenario is taking away Visitor Messages or not letting people post in threads. It's funny, because we've only had VMs for about a year and two months now, yet we've become really reliant on them. I don't think we'd be able to deal with Private Messages anymore on a daily basis, because VMs are just so much easier to operate with. However, if we weren't able to post, the entire point of the forum would be void. That'd be pretty bad too.
Being unable to use custom avatars or signatures would cause major disruption as well. If PC's known for anything, it's creativity. Some of the themes I've seen here have been works of art, and taking that sort of stuff away would be taking away from someone's personality.
Hm, what else? Blogs are awesome too. I spend about half an hour each day reading the latest entries because they're normally all interesting. Losing the forum skins would be unforgivable too, now that I think about it. PC wouldn't be the same without Unseen Forces or Forever Standing.
And.. that's all I can think of for now.