1. The simplicty. The efficiency. The feedback from the drivetrain/tire to surface. I'm a nitpicker about the stupidest things and derailleurs provide nearly limitless potential for tweaking and diagnosing, and me, being on the lighter-side of OCD, would spend lots of time just fussing with them. Besides that, geared bikes, even the nice one I have, feel like slugs now - all sloppy chainline and mysterious noises and such and no matter how fast i'm going, they still feel slow and disconnected. I've still got a couple of geared bikes (Specialized Sirrus and some kind of Panasonic grocery getter) but my fixies get most of my attention.
Esthetics fits in there somewhere, too. And I always run at least a front brake (for my safety and yours).
2. Pure and simple peer pressure. Actually, it seemed to be the only way to get Gurgus to shut the hell up about fixies and actually start riding one.
3. Rigid MTN bikes, assorted POS's. Usually a bike that starts out as one thing but gets re-purposed for another. The Sirrus is the most modern, complete, and well appointed bike I had bought and its still a good bike despite losing 16 of its 24 speeds (on purpose - front derailleurs, BAH!) but most of my stable is cheap/free bikes lovingly/frighteningly restored.

Dave G.