In no order - all 3 are equally important
1. Ease of carry - Can't be too bulky, can't be too small. A light you never bring with you isn't much good.
2. Battery life/output relationship - ability to conserve battery life with a low output setting is ideal, as it is not often you need the upper end of a light's potential. Overall battery life therefore is a consideration or you are always changing batteries to make sure it stays reliable.
3. Build quality. I bought a Surefire G2X Pro after a sales person produced one, threw it to the ground as hard as he could, then used it around the room. Lights get dropped, get forgotten in vehicles, get knocked around, etc.