I don't care for the fact that Tumblr seems so... public. I don't think you can really lock things down or filter like you can on LJ or FB. The reply system certainly leaves something to be desired, anyway.

(I occasionally remember to check Tumblr anyway, but I only go back five pages, and it only occurs to me about once every three months, so I see almost nothing that gets posted there. I just really don't like it.)