Mostly price tag, functionality, and looks. Cheap knives are fine, as long as they're relatively functional. No replicas/fantasy knives, I'm likely to avoid anything where I don't trust the lockup or blade strength, anything that seems to be heavy on blade play, that sort of thing will generally rule out a knife for me, cheap or pricy. As far as pricy goes, so far I have yet to find an expensive knife whose features are enough superior to the features of a $200(or less, for that matter) knife to warrant the extra price. That said, then comes my other thing-looks. If a knife looks great, I might consider paying the extra price for it even if the features themselves don't justify the extra price...so far though, I haven't found any production knives that look good enough to pay more than $150 or so(the Cold Steel Espada XL comes close, despite NOT being worth that price from a materials standpoint). Seen some nice customs though, and I'm sure one day I'll get one. On the other end of the spectrum, if I totally dislike the looks of the knife, I'll likely not own one either, again, no matter how great or cheap it is. If someone wanted to sell me a genuine Sebby for $10 on the condition that I HAVE to keep it and can't sell it, I wouldn't be interested. So basically, being very junky will drive me away, being ugly will drive me away, and being too expensive will drive me away unless the knife is aesthetically appealing enough for me to consider overlooking the price. Oh, and no counterfeits.
Other things don't bother me. Assisted open is fine for me, I like them actually, but I won't shy away from non-AO's either. Likewise, I dislike opening holes(give me a set of thumbstuds ANY day), yet I am totally willing to buy Spyderco and Byrd stuff. I prefer plain edged knives to serrated, and prefer full serrated to combo edge, yet I own knives of all 3 sorts. I prefer tip up carry, but I own plenty of tip-down only models as well. And so on...