Conformation first and foremost (long, deep and thick body with good legs). I want calves that inherit the type of body structure that the buyers are looking for at sale time.
If purchasing a mature female, udder structure all the way. My cattle's summer range is a pretty big area, so good feet are also important in both males and females. Always look for females that are wide through the hips...broody looking. I hate having to pull a calf, and can help cut that down by buying bulls with reasonable birthweight, clean fronted long body, and keeping and eye on EPDs, but if the cow is narrow through the pelvis she probably is going to end up eventually needing an assist. Murphy's Law sort of thing.
This is probably going to sound redundant, but to sum it up:
Feminine females and masculine males.
I've ended up with the odd cow that was masculine looking and they ended up back on the truck for being poor cows...both in the milk department and fertility wise. And Honey once bought a bull that was a little on the fine side of things, and the only breeding he did was to the other bulls. Cows would cycle and he'd just hang around doing nothing until another bull showed up, and then he'd be making a sandwich out of the other bull when he was riding the cow. He grew wheels, but ultimately Honey lost money 'cause a dud bull in summer is surely worth a lot less than the same weight animal at a bull sale in the spring.
Take care.