Now, I'll make it clear, I don't think that LiveJournal is going anywhere, but it's possible. It's possible that some act of God might physically wipe out all the servers. It's possible that someone with a lot of money and some crack lawyers might decide they didn't like LiveJournal. It's possible that bradfitz might have a nervous breakdown and rm -rf / all LiveJournal servers. You just never know.
So with that in mind, what would I lose if LiveJournal disappears of the face of the earth? The two most important things, I think, would be the archive of my entries, and access to people I know on LiveJournal. The fromer wouldn't be too bad, because I've downloaded most of my entries, and do so periodically. It's a crappy format, and I'd lose a few recent entries, and it doesn't download comments, so i's not ideal, but that would take away a lot of the sting. And I only occasionally want to look up things from the past anyway, so that's not too bad.
As for contact with people, I think that most of the LiveJournal people who I would miss, I have means of contacting them outside of LiveJournal. I rely rather too much on @livejournal.com email addresses now though, so that could be highly annoying. i ought to compile a list of email addresses of my friends list at some point.
In terms of how much I'd actually miss if it was gone, I don't think it would be too bad. LiveJournal isn't a terribly important part of my life any more. It's something that I do, but nothing I coulnd't live without, or get used to being without. I'm not so accustomed to writing things onto LiveJournal that I'd accutely miss it if it weren't there. In a way, I think I'd be glad. LiveJournal somewhat encourages a one-size-fits-all approach to information excahnge and friendships and I rather miss the days where friendships were just one on one, and if I wanted someone to know something I had to tell them. It would also be somewhat fun to get into using some other web logging system. On the other hand, I would definitely miss the aggregation of the friends page, but it would be an excuse to get to play with RSS aggregators.
Overall, I think I'm fairly well set. It's something interesting to think about though, and I rather suspect not something that most people consider.