Quality of a sword in general has two parts
"How does it look?" and "How does it perform?"
Depending on the person, one or the other might be more important.
"How does it look?" encompasses not only the beauty of the blade, buts its history, maker, and adherence to tradition and is based on personal feelings which can vary wildly from person to person. For example, an Angelsword
katana is pleasing to the eye, but it is not traditionally mounted, so a more traditional kenjitsu student would not value it as highly as say, a Swordstore
(very traditional) blade, while another person would prefer the non-traditional mounting. Also we have bragging rights. A blade Howard Clark hammered out in an afternoon will be worth more than Joe Smith's masterpiece simply due to reputation.
In "How does it perform", we look at the quality of its actual construction. Does it have an even temper or differential, and are the temper lines even? What sort of steel is it made from? Where is the point of balance? How is the hilt constructed? How hard is the edge? How well can it cut? Other things can only be determined by destruction testing: how tough is the blade? How much force can it take before it snaps? These are tough to determine and data is easy to fake, which is why peer review is essential for buying swords.
I hope I have been of help.