Nice thread. If i ever play with a traditional "number 10" type player (very attacking playmaker amc) i always try to give him 21. I'm not really sure why, i think it has something to do with pablo aimar back in an old CM game but i could be wrong.
I also have a massive prejudice against any player once he turns 30, it's like a switch for me, <30 fine >30 too old. I try to ignore it in most games but any time i have the money to replace a player over 30 with one under 30 i can't help myself.
The only other one i can think of now is a friend of mine who i play with a lot cannot have the mouse cursor visible on screen during a match (he hides it off the side of the screen)
keep em coming, it's interesting to see all the weirdos out there!