One is the hole. I infinitely prefer the hole to any other way to open a knife. The hole is pure genius. It simply works far far better than a stud. Case closed.
Two is the sharpening. Spyderco takes pride in sharpening their knives and they do it properly. Out of the box, Spyderco knives tend to blow me away. Many (most) other makers are shockingly sloppy about sharpening their otherwise superb knives. Don't ask me why. A sharp edge is what a knife is all about.
Three is the attitude. I am always amazed when Sal from Spyderco contributes to forum discussions. He and his son Eric clearly love knives. For them it goes beyond making money.
My favorite Spyderco is either the Manix 2 or the Paramilitary 2, depending on the day of the week, phase of the moon, and other factors.
Knifeworks is where I got one Spyderco Exclusive (a military with CTS204P).
BladeHQ supplied my next via a waiting list (the Manix-2 CF with 154CM and S90V).
The CP is new, and offers more of what we want in a knife. It's tougher, stronger, has more color capability and is not significantly more expensive.
I have always liked liner locks, but I guess just for sentimental reasons. I hear people bad mouth them a lot. A good one is just fine and when it fails the blade may not close on the user (as in the test above). Any lock poorly done is a hazard.
Tom's Knife Info / email@example.com