When you get into a conversation with someone in a thread it's really difficult to follow it when you're quoting, the conversation gets all over the place and other guys can't really dig into it and bring something to the table. A replying capability very much to the style of facebook and 9gag would really help get the conversation going.

That defeats the whole purpose of it being a forum

That would be true if you turn the forum into a social network such as facebook, but just implementing one the tools they already have doesn't kill the forum concept. It actually makes communication way easier.