I used to have some definite turn-offs, but I'm more open minded now.
A useless choil will make think twice. I think Ed Fowler really made me notice choils more. Thanks Ed.
Fantasy tacticool blades with built in wire-cutters, bottle openers, and microwaves will not make it through the door. This rules out just about every Gil Hibben knife ever made.
Saw teeth are a big no-no, but I could be convinced I suppose.
"Chisel grinds." This tells me the maker doesn't understand knives or is lazy. Or both.
440C. I know I should be more open minded here, but too much crap 440C has turned me off completely. A good knife maker could probably change my mind.
I used to mind bad quality sheaths, but now there are enough good after-market sheath makers that I can over look a bad sheath if I really like the knife.
Thick steel used to make me think twice, but I have a couple .25" blades and they work well. Still something more reasonable always seems more...uh...reasonable.
I am not dogmatic about blade design. Each design has a strength. I love the look of a clip blade, and they are functional. Upswept doesn't do it for me, yet one of my favorite knives has a slightly upswept blade. I love a Wharncliff--the antithesis of the upswept blade. Go figure.
A big ricasso used to annoy me, but even here I am willing to tolerate one. Most cutting takes place further out on the blade anyway.
Currently the thing that catches my eye is the stress riser. A knife with a curved, soft stress riser looks like it was well thought out. A sharp cornered stress riser causes me to wonder if the maker thought about it and why he/she didn't eliminate it. Makes me hesitate.