1) To know that they're mentally there. I don't want to have to mentally censor myself, either because I don't think they'd like what I have to say (in the case of philosophy) or because I don't think they'll understand (in the case of technology). I don't insist they be on the same level I am with computers, but it would be very nice if they were close enough that I could explain complex stuff without trouble.
2) To know that they're physically there. Someone who won't avoid me for no obvious reason, someone who's at least reasonably sexual and physical.
3) Symmetry. I don't want someone I'll just run over, I want someone who's motivated and fascinated by life also. I want *her* to want to discuss cool new things she's learned or figured out.
(3) is relatively common in that I can think of half a dozen people who could fill that role, and probably more that I'm not thinking of. (1) I've had twice at most. (2) I've had once at most, and that's stretching it. Naturally these are all entirely disjoint - I have never had more than one at a time. Never. Unfortunately there's no "well, X is preferable over Y" going on here - these are all showstoppers.
Combine this with my current amazing ability to trust, i.e. the fact that I can't. Last person I opened up to at all was about a year ago, and I've half ended up regretting that. I don't see it happening anytime soon, and I've got enough emotional baggage that I don't imagine any relationship could work out at all without that. (Ignoring the question about whether it could anyway.)
Basically, I'm fucked.
That's really all I had to say.
Oh well. At least I've got my life work.
I suppose I should make it doubly-clear that for (1) I'm also referring to being able to discuss deep technological stuff, and that I'm not even counting you on this chart if there hasn't been a reasonable period of physical proximity *and* possibility of some sort of a relationship. So if you're someone I've never met in real life for more than a few days total, don't go trying to figure out which category I've put you in :P