Oh, so many things. Naming just one would be really difficult.
The cost/benefit ratio is number one for me. If I feel that I'm paying for the name on the blade, or some "limited edition" nonsense, that's a stopper.
If it checks that box on the list, there are a bunch of other things that need to be right.
Ergonomics is next for me. The most amazing blade in the world isn't worth anything if it hurts your hand nearly as much to hold it with the right end, as it would the wrong end!
Heat treat is next. Kershaw used to use the same 8CrOvM17A8n64... whatever it is, as CRKT, but they did a proper job of thermal cycling it. Don't know about anymore, but the two plain-jane Cyclones that I have are pretty good for the price.
Edge geometry. Much as I love sharpened prybars, the job of a knife is to cut things, and if the edge bevel is too obtuse to do that, then it ends up being more of a novelty piece that I keep for those "hard use knife" threads....
There are other things: colors, pivot screws that need a dedicated tool to tighten, serrated blades and recurves are not my favorites anymore, and on, and on.
I believe that if you think too hard about what features you like and don't like on a knife, you end up in a similar situation, where it's easier to talk yourself out of a knife purchase, than into one.