It's sort of...closer to Livejournal in some ways than Tumblr? It's based on communities, rather than personal accounts - which confuses a lot of people who are used to both on LJ. But you can basically set up a 'community' that is just your personal journal, named after yourself. It has threaded comments and lots of things LJ has that Tumblr is missing, and like I say it's very community based - which I've missed lately - so lots of people have joined it and are hoping it takes off. I think you need a code to join at the moment though.