Nebari(Surface roots)-Should radiate outwards from the tree , look for ones with similar sized roots instead on ones with one big root and a few weaker ones..
Taper-I doubt you find anything with good enough taper to just start growing brances off of in a nursery, so try and find something that atleast doesn't have reverse taper down low, or look for low branching that could be used as a new leader once a chop is made..
Movement-(Straight IS BAD!!!!!!)-Look for something with a subtle undulation or wiggle to it in the first ten inches, not extreme(like an S shape) but nice and smooth, natural is the word I'm looking for here...
Grafts-(BAD!!!!!)-Some people say look for a low graft( so it isn't noticeable), but I , for one, cant stand grafts so I normally look for a higher graft and then air layer the cultivar scion off and ,Viola, you have two decent trees on their own root stocks and the original root stock actually has a trunk....
That pretty much sums up what I look for when I'm digging in a garden nursery for Jap Maples......I expect other's methods will be pretty similar....
Oh, one more thing
SKINNY IS BAD!!! THICK IS GOOD!!!