1- balance. to me this is how the bow feels in my hand. I tend to prefer bows that are light at the tip. it has been harder for me to find one that sounds great though. In general I have found that the ones that are heavy at the tip tend to sound better on my bass, but they feel unfriendly to me.
2- evenness. I play open strings very slowly and try to notice how the bow behaves on the upstroke and downstroke. This tells me how much effort it takes to control the bow in order to obtain an even sound.
3- dynamics. when I play a bow that I really like I feel like i can create dynamics that go form ppp to FFF and back very evenly on notes as short as a quarter note!
4- spicatto, (bounce), some bows feel real heavy on the string, and some feel really light. I have a bow that does this freaky thing, when i am going downbow and start to get near the tip the bow starts to bounce like crazy!. it takes a great deal of concentration to keep that from happening. Anyway, bow bounce differently and i suppose you will choose the one that makes you play better.
5-sound, some bows tend to sound airy and hollow, other sound very heavy, muddy, quick, dull, sharp, sluggish, fast, uneven, etc. etc. and although the descriptions can be sort of abstract the feel on your hand and the sound in your ears are very real.
6- string crossing, how easy is it to cross strings? does it take a lot of bow to get an even sound? is it smooth going from the low E to the g string? does it take a lot of concentration to control the bow? etc.