Best qualities in a mechanic? A masters degree in mechanical engineering, and several years of floor-level experience in manufacturing/fabrication. Bike shop experience may not even be necessary, as competency starts with a firm grasp of the fundamentals. A professional engineer will have a deep understanding of materials, statics and dynamics, lubrication, corrosion, finite element analysis, CNC fabrication, CAD/CAM etc. that you could not gain by working several lifetimes in a shop.
The best bike mechanic I ever came across had this combination of education/background, and despite having no specific bicycle experience, within 2 weeks of starting to work on bikes, he was able to do things that most professional mechanics could never figure out. Which included identifying flaws in brand new components, and fabricating improved custom replacement parts.
The sorry bicycle industry cannot afford this type of person, particularly at the retail level, so 'competent' is probably as good as you can expect.
Worst bike mechanics? The barely-literate bike flippers that clutter up Craigslist with dangerous dysfunctional crap that they 'fixed' or 'tuned'. I've ran into many of these folks, and despite perhaps decades of experience in 'working on bikes', they don't have a clue.