A thorough understanding of how to control mud formation and refinement on synthetic and especially natural stones. Knowing which sequence of natural stones will give a specific level of contrast and how to modulate the contrast levels and bring out patterns in the steel.
Able to produce your first burr when the two sides meet at 16k grit - at will
Able to discern the difference between an 0.025 micron edge and a 0.1 micron edge by touch.
Able to change a single bevel angle by 0.2 degrees - without messing up the shinogi line.
Actually understand how asymmetric edges are created with a numeric level of understanding.
Ability to freehand, use a belt grinder, edge pro and Wicked Edge.
Ability to do convex edges on precision devices like EP and WEPS. Also progressively increasing or decreasing edge angles from heel to tip.
Ability to use all sorts of naguras - tomonaguras, hybrid slurries, true naguras and even odd naguras like Jokeji.
Ability to sharpen all sorts of knives including serrated edges, recurves, etc. Also straight razors.
Able to refine a hamaguri grind's radius of curvature by a controlled number of degrees.
Able to thin a knife - not just the bevel - without rubbing off the Kanji.
And most important - the ability to determine what sort of edge is most appropriate for the knife user - be it your customer, or your own knife. This involves interviewing your customer and understanding his needs, not yours.
A broad level of understanding of compounds, strops, belts for belt grinders, the stones available on the market, Japanese stones, European stones and other stones - including their geology and characteristics and range of available characteristics for a given stone type.
The ability to 'see' what sort of edge you are getting in your 'mind's eye' - and confirm it with a scope.
This should make this a little more challenging