Everything's pretty much been covered... 4 teats about the size of your thumb, cylindrical and planted squarely on the udder.. if it's FULL of milk, I'll say the bottom of the udder at the same level as the navel, if it's not, the bottom of the teats at that level, though if the cows are dry, it's pretty hard to tell, unless it's already far too big.
I think if you have range for your cows, a bit heavier boned is good, and perhaps a little taller legs makes them a more mobile. If they're going to be on nice pastures, I like to lighten the bones up a bit and bring down the size. Again, if you have lush pastures, look for breeds that milk heavy, and likewise if you graze bunchgrass, it may not support the big producers as well as it would a more modestly producing breed.
Nice toplines are nice to look at, but the bottom line is the one that makes it work.. I like them to look long and cylindrical when viewed from the side, and not pinched at the chest or the waist, and the udder at that level as well. WIDE hips, certainly no hatchet-butts, that helps you sleep at night during calving time. We're talking about beef cows here right? They should make your mouth water when you think of the steaks on those loins. Since I got a Gelbvieh bull (breed pimp warning), looking over the backs of the cattle has changed... I used to see a sharp ridge (the spine) all the way from the neck to the tail.. now, from the shoulders back is flat and wide, and the spine is below the muscle level.. Also, what looks like their hips, isn't bone, but muscle over top.
Hooves... They should look like they walk on their toes, not their heels.. otherwise they won't wear the tips of the hooves off and you'll end up having to do it.. For the stride, they ought to put the hind hoof in the print of the front when they walk at any speed faster than grazing.
I'm with Branguscowgirl on this too.. if they hightail it when you come to their pen, you should too..., though there are wild cows that are OK with humans around their calves, but tame cows that will kill... hard to tell the two apart, whatever the case, pay attention to their body language at calving.
Oh, and they have to be red!