My first qualification is that it's hypoallergenic and non-porous: Glass, stainless steel, silicone, NobEssence's wood. I've got pansy bits so I have to take care of them.
My second qualification is that the shape is unique, or different from something I already have.
If I'm not sure about the company, reviews have to state that the construction looks like it will last, and that it's really made of what it claims to be made of.
I will also push things way down on the list if they are pink or purple, and since I've amassed a lot of black without realizing it I'm starting to be skeptical of black too.
I don't even consider size, I guess. "No worries, I can make it fit!"