Most important technical aspects of speaker wires? Capacitance, inductance, etc...

Looking into speaker wires, and technical specs or construction stated by companies, has anyone determined, measured of figured out WHAT are the most technical parameters for picking speaker wires? Is it Capacitance, inductance, gauge, insulation, construction, material used, shielding, etc...?

I do not buy thousands of dollars in speaker wires, specially for my $5K setup, but within reputable speaker wire companies being Furutech, Cardas, Audioquest, Nordost, etc... how would one pick wires? Obviously listening is the only real solution, however, many have to be broken in, not everyone has the options of trial, and it's a difficult task to complete if you're looking at multiple manufacturers, and various brands within each.

One last question, the outer sleeves used on high end cables, are they at all necessary outside of looking nicer?