Enough popularity / reputability, that one will want to run the software
and finally: Not WordPress
Serendipity seems to have all 6 of the above. It is a pain to install the way I did it. I'm mod_rewriting the /serendipity/ directory away and it causes a ton of pain. I managed to do that, but it is obvious that the authors did not plan this software to be (ab)used like that.