It's a code fork of Livejournal, but they're taking the opportunity afforded by starting up anew to rip out large chunks of code by the roots & rewrite things *better*. The biggest example (for me, anyway) of this being the replacement of the friends system by a system which explicitly separates "people I want to be able to read my stuff" from "people I want to read". For me, that's pretty much the Killer App right there. The increase on the comment limits from 4,300 char to 16,000 (there's a similar post size limit increase, but I've never hit the post limit so that bothers me, personally, less :) ) is also shiny. Plus: no ads. And one of the owners (synecdochic) has discussed at length why ads on social content sites Do Not Work, financially speaking, so I believe that they actually mean this.
The meta-things that I'm impressed with:
- a commitment to openness (of business practices and site-running) and consultation that I believe in. Possibly in part because I've been involved with and/or witnessed discussions about proposals or ideas that have resulted in decisions being changed :)
- a diversity statement that doesn't read like corporate bullshit.
- the business plan is along the lines of "small, stable, functioning, ongoing business" rather than "let's flog it in due course & bathe in web2.0 cash". An interesting post on the subject of business aims & so forth.
So, yeah. I shan't be moving off LJ altogether unless/until various things change, but I'm certainly going to be doing a certain amount of moving across. I gather there is a crossposting tool, for starters. I'm very fond of LJ, but only in the sense of being fond of the things it enables me to do. DW feels like a project I can be fond of in a more specific sense, and I like that.
 The real Killer App would of course be full interoperability - i.e. being able to read one's LJ friends-list, even locked posts, on DW via OpenID. This is a Difficult Technical Problem for which there is at present no decent fix, but it is being contemplated. Full import of your own LJ will be in place before open beta; OpenID already allows cross-site reading & commenting on non-FL posts.