I'm an opener, so my opening partner is critical to me. They need to be willing to face the first over, be of a similar pace to me (I had to run with a fat guy last year, and he simply can't scamper between them like I can. Quality bat though), call well, not hold down the scoring by being stagnant at the crease, and (this bugs me, playing at junior level) simply not be (No Swearing Please) stupid. Bring some intelligence into your game for gods sakes, don't go after the ball when you're a number 11 and you need to see out another 18 overs.
^ This as well, being part of a left-right combination and running quick twos that turn into threes is awesome fun. Gets the opposition in all sorts, then you can simply smack the ball into gaps. Best fun in the world, honestly.