No, you're not missing anything, and I don't think that LJ will be replacing GreekChat anytime soon. LJ is a site when you can have communities and keep your own online journal. The beauty of it is that you can keep your journal or communities as public or as private as you want.
For me, I've always wanted to keep a written journal and have never had the discipline to do it. But I've had my LJ for well over a year and I still love it and use it everyday. LJ helps me keep up with my sisters and friends' lives. Plus I don't have to worry about every Tom, Dick, and Harry reading my entries like I do on other public websites!