Hm...
Probably the biggest deal-breaker in me might be that I'm full of extremes between which I alternate quite a lot. So usually on the beginning, when I start to like someone/fall in love with someone, I'm all "I want to be with you as often as possible, prefer your choices over my own, do almost anything in my might to make you happy", and over time it slowly changes to "now you're more like a part of my life, and I sometimes need quite a lot of time on my own, I never wanted you to adapt to my expectations, and I'm not going to adapt to yours, because I'm a free being as much as you are, I sometimes prefer other things instead of being with you"...etc. I know that this is probably a common thing, but most of the girls I dated didn't react that way, so either there is something uncommon in how it occurs with me, or they were not "mature" enough, I don't know.
Then, I'm often pretty lazy and passive type, so it is hard to get me to go somewhere or do something. Also, me organizing anything? Sometimes I'm literally scared of that, including calling someone to go out because then I feel responsibility and pressure that I have to make it worth their time, otherwise I just wasted it, and sincerely, I have no idea why exactly would people enjoy my presence, even if they look as they do, it's hard for me to believe that.
Next, I'm a geek... kind of. Not that kind of geek that socialises with another geeks, but that kind of geek who usually nothing to talk about even with geeks. It seems my way of thinking and percieving the world is very different from anything "normal" (meaning "mainstream"), but also different from anything generally considered different. And it usually turns out, sooner or later, that my ways are pretty much incompatible with everyone and anyone.
...NEXT (yeah, there's a lot of things) I'm struggling with practical life quite a lot, such as I've got problems with concepts of responsibility, duty, getting on time to places, I usually don't have money, etc...
Next, I love to analyze things very deeply, and talk about my thoughts and feelings a lot, which to most people appears as if I'm bragging about how smart/intelligent I am, or just plain narcissist.
To cap it off, there seems to be an intrinsic incompatibility with girls I usually choose to date, because I don't "choose" them based on similar interests or personalities, but exactly the opposite, as I am intrigued and fascinated by personalities and minds which I don't understand, and I'm drawn to try to understand them as deep as possible, which most of the people allow you only when you get into relationship with them. And sooner or later this comes back and bites me in the ass.
Generally, I'm a difficult person to be around, requiring immense amounts of patience, understanding, active will to understand, and most of all, being accepted the way I am. Sounds to me as one of the worst possible combinations.
Also, I tend to be VERY sincere, as in whatever crosses my mind I say, and I love when I have the freedom to do so, but usually after some time people grow tired of it and/or start to hate it, at which point I start to censor myself, and the number of "specific things I need to not say" slowly grows, and upon reaching certain threshold, I feel caged, the relationship looses the meaning it has for me, and I back out, or resign and start to be sincere again to make the other person "deal with it or back out".
(captcha: ladies first. fuck this :-D...
and it says I put it in wrong. the next one is: it's over. yeah, it is, for about two months now.)
Yeah, also, all the good attributes I have (there's probably not a lot of them) I automatically expect/need the other person to have too.
Edit: Oh yeah, I also have my arrogant streaks when I think that my opinion is
a) self-evident and logical
b) good
c) so stupidly simple that how the hell can you not agree or at least understand it, you stupid, primitive pitiful being, do you know how it insults me to share the same species with such an individual as you?
But this happens only with people I don't really know or care about, so my girls are affected only indirectly when they witness one of these and get angry/embarassed about how arrogant I was.