I hit submit too soon, because after I did, I saw that I'd forgotten to check the one I'd rate as most important:
Brand/maker Reputation

I'll rarely buy a knife unless I can find information about the manufacturer/maker online, and they have a good reputation. Also, I never buy from a vendor that I can't find positive feedback for anymore (and not just on their own site) -- that's not knife specific, but is a warning worth noting to avoid getting taken or having someone rack up fraudulent charges on your card.

I selected (in order of importance to me, after Brand)
Other
Overall looks
Shape/Profile
Steel used

For me "Other" is an intangible -- primarily, it's my momentary feelings about my finances. I've passed on deals that I really regretted because I didn't feel that I had the discretionary funds at that moment. At other times, I've bought knives that I later decided were absolute junk, simply because I thought they were interesting at the time and I was flush -- these were usually less than $50 buys, but some were more and they add up to quite a bit.

Another that I didn't choose because it's not a "deal breaker", but how "Rare or collectible" a knife is often a big consideration for me -- for example:
A couple days ago, I came across a BladeForums Exchange listing for some older Al Mars (PathFinders sold immediately or I'd have grabbed them -- I've long admired their khukri-inspired design). However since those were gone, I snagged an Integral and two classic SERE fixed blades -- all three having the same SN as a matched set. The prices seemed good ($420 for those 3), but I'm not an Al Mar collector -- these will be my first. I suspect the steel is AUS-6, which may have been great in its day, but it's no super-steel by today's standards. This case was a bit abnormal for me, but it was all about knives being collectible rather than passion for the brand or those specific models.