Pillowfort is a lot like Tumblr (easy to post images and extensive text, easy to repost to another journal, discoverable through tags), with threaded comments on posts (no follow-up like Tumblr), plus communities that can host both posts and discussion topics. You can get an invite by sending them $5 on PayPal. Right now, they have some serious scalability issues, so I think they’re right to be in closed beta.