Ease of use: The easier a call is to get comfortable with, the sooner it can be put to good use. Mouth fatigue also plays a part in this—the less effort a call requires to blow, the more effective it will be.
Sound quality: No animal in distress sounds "pretty," but an experienced ear can tell what sounds real.
Volume: On a blustery, windblown day, a loud call can mean the difference between going home empty-handed and a skinning party.
Versatility: A call that can make multiple sounds means you won't need to switch to a different call as often, making for less movement and noise that will give you away.
Freeze factor: In colder regions, having a call freeze up is a real concern, and more than one call has been slung into the brush because it froze at an inopportune moment.