Yeah, I don't find it particularly useful as a social networking platform because of the lack of interactivity. When I hear people talking about fandom potentially leaving LJ/DW for tumblr I just wonder...how?

As a general blogging platform, the lack of comments (or at least ease of commenting, since I haven't figured out if you can comment even with an account) does hurt it. Comments are usually where I find the most useful information in a blog. People are still flocking to it, though, so I suspect for many users, the lack of interactivity is not a deterrent and might even be attractive.