I primarily go for looks. Genuine, natural materials on a sleek frame are features for me. Looks includes the blade shape as well. Three of my favorite shapes are the Buck 110, the Spyderco Native and the SOG Aegis.
Second would be ergonomics. If the knife is not comfortable in hand, then what is the point? Once again, the Buck 110 is the most comfortable line I own.
Third is the steel. Anything over and including 420HC is fine. I have Buck 110s' in 420HC, 425M, 440C, S30V, CPM-154 and BG-42. All will do the job.
Fourth is customer service. If a company isn't willing to 'have my back' with their product, I just won't own it.
For me, it is not enough that the tool performs well, it has to look good doing it.