The archiving facility is good, but a lot of people (myself included) would probably like the option for a much simpler form than CSV or XML, namely as mere HTML.
So an archived post would look something like:
May 17th, 2001
This is a post... yaddayadda
Current mood: hot for archiving
Current music: something funky
I.e., very simple, not of any use for recreating the journal with lots of LJ internal data, but merely for reading in a browser, with text formatting, images and links preserved.
Edit (April 4, 2006): This suggestion was implemented independently in jbackup in 2004. Hooray!