A simple answer is quality, brand name, pattern, size, cover material, and condition. There's a more complex discussion in many topics in Bernard Levine's Identification subforum.
The understanding of condition can vary and influence desirability. A knife that is "cleaned" or buffed is not new but some folks mistake it as new. Sometimes the knives that are in worse condition and have been "cleaned" are desired over knives that are actually better preserved and original. This confusion seems to be extremely common. There are examples in the "porch sourcing" topic.
The herd like behavior of the consumer has a large influence. People want what others want/own. Something that is of extremely high quality may have little desirability if it is not well known or popular. Even two identical knives made by the same manufacturer can have different desirability... for example if one has a well known brand and the other has a lesser known contract brand.
