Frame-lock flippers that close one-handed like glass without having to overcome a jarring speed-bump. (Thank you Les George Myna)
Good liner locks.
David Mary
mentioned it earlier.
Grip designs that keep your index finger securely close to the cutting edge without using a finger choil on the blade - exceptions for small knives that make perfect use of choils to account for overall length (ex. Chaparral and CS Tuff Lite).
Grip designs. So much of what boils down to when I absolutely love a knife, like it, or hate it, comes down to the grip. Feel in-hand is something you can only experience when you use a knife, and for me, it trumps so many other attributes in what ends up being my favorites. You can't truly see it in a picture, and you can't put it on a spec sheet, but when you feel it - you know it.