While most answers you are going to get are highly subjective and reflect the individual's opinion of what goes into a well made knife, there are some definite things that are consistent and required that go into the making of any quality knife, independent of opinion. The biggest is getting the details or small stuff right. Fit and finish is a huge indicator of quality. Put another way, the majority of knife makers can get their product to 90% level of professional standards. That is fairly easy and generally gets done in an amount of time and costs that allow for profitability. Raising your product that last 10% can take 2-3 times more time than it took to get to the 90%. And many won't bother. The top producers do bother. They sweat the details.
But there are also makers out there who won't bother getting to 90%. How to spot them? Do the parts line up? Are there small gaps between the frame, or the tang and the scales? If there are, that means the maker didn't make sure the scales and metal were both perfectly flat before joining them. How is the edge? Is it sharp? You would be amazed at how dull some knives come off the production line. Is the finish smooth and even? Any rough spots? Sharp edges that aren't the blade edge? Color differences, scratches, pitting, cracks, in the scales? The Layers? The Blades? If it is a folder, how is the W&T or does it flip open smoothly? Is there lock stick? Does the dent feel right? Does the Sheath quality match up with the knife quality or does it seem like an afterthought, or the place that the maker made up his profit margin?
Is the knife well made for it's intended purpose? If you bought a filet knife that has zero flex in the blade, you bought a poor filet knife. Is the choice of steel used, a good choice for it's intended use? If someone tries to sell you a saltwater dive knife made of O1, run from the deal! If the intent of the knife is to be a modern EDC, then it should generally come in one of the premium steels. If only because that is what the quality knives have right now for modern EDC. A top quality traditional knife can have a steel like 1095 and be top notch. A modern EDC, not as likely to pull it off if only because it is not SS, and as high of edge retention. For the category, modern EDC, it needs a SS Super Steel because that is what the market is requiring to be considered a good quality modern EDC.
The flip side of a lot of this is that you can't just use the type of steel the knife is made out of, as a determination of quality. Buck uses 420 steel in a lot of their knifes. 420 is generally a low end steel. But Buck's heat treat process squeezes every bit of performance out of the steel that is there. They make a quality product with a low end steel. There are plenty of examples of poor quality knives made with super steels. The manufacturers know consumers often make a buying decision based solely the steel. Expensive steel? Has to be a quality knife right? No, it only means it is a more expensive knife. The quality never showed up in the overall construction of the knife. A crappy knife in M390 is going to sell for more money than the same crappy knife made out of 440A. Good way to improve your profit margins without having to mess with things like fit and finish.
How does the knife compare to other offerings in its price range? Mora makes a high value, well made product. Is a $15 companion, just as good as a $300 Fallkniven F1 Pro.? No. But they are BOTH well made knives. The bang for the buck comes into play here. You can't expect a $30 Kershaw to have the fit and finish of a CRK. That is not who you should be comparing it to. How does it stack up to other $30 knives out on the market? Is it better than the majority out there? THAT is a determination of a well made knife. You can only put so much into making a $50 knife. before it become impossible to produce at a profit. There are companies that produce a truly great well made $50 knife. Don't compare it to a $1000 custom job. Although some $1000 customs may not bear up under the scrutiny.
Some things that ARE NOT indications of quality:
edge retention - There are a host of factors that go into producing a high edge retention. The biggest of which is what was the manufacturers intent. Did they intend to make a blade that held a sharp edge for a long time. Great, but that is not the be all end all. Some makers think their customers want a knife that is easy to sharpen, and don't have to go out and buy 2-3 diamond stones to put the edge back on their blade. Some consumers prefer to have a knife that comes back to life after only a few passes on a Arkansas stone. A quality knife will have the edge retention in line with the purpose and intent of the knife. Edge retention, and steel type will be carefully weighed with blade design, edge geometry, heat treat, knife style and purpose. It is a big balancing act and a quality knife maker will take the time to make those proper choices and test it out.
The feel in your hand - entirely subjective. I just bought a beautiful Hess Bird & Trout knife. Well made quality product. Can't get all 4 of my fingers on the handle. Everyone's hand size and preferences are different. You can have well made that doesn't fit you but fits perfectly on someone else. Not an indicator of quality. Likewise, the "heft" of a knife can be deceiving. A heavy or substantial knife maybe viewed as well made, hence the trend in overbuilt folders in America. But a really light knife can be a thing of beauty and be perfect in every respect. Top quality it not determined by weight.
The design - By that I mean the general look, appearance or ornamentation of Zero indication of quality. That part of design is all about catching the eye of the consumer. For further info, see "Gas Station Knife". The other aspect of design, constructed for a purpose is a indication of quality. Bob Loveless' Drop Point, Opinel's ring lock, Grohmann's Canadian knife are 3 examples of stunning design, strokes of genius in providing the desired results of the knife maker. In the 3 mentioned, the benefits are respectively - ease of dressing game without cutting into the entrails, Simple effective design that lowered production costs, balance in the hand and ease of slicing.
Price - Some very expensive knives have some very inexcusable flaws and defects. As the German's say, "All that glitters is not gold".
The Guarantee - They can say what they want on the package. Getting them to honor it is an entirely different thing. You actually have to look to find a maker that doesn't have Limited Lifetime Warranty on their products. OF the 100+ knives in my collection right now, I have 2 that have a 2 year warranty, 1 with a 5 yr. and one more that is against "manufacturers defects". All the rest to my knowledge are lifetime or limited lifetime warranties. No I am not going to name the 4.
