Wire insulation can be a very important factor in your motor. The wire insulation protects from shorts in the wire in two ways:

1. Abrasion resistance - Better abrasion resistance decreases the likelihood of a short when winding the motor. If your motor has chips in the stator insulation or you tend to rub the wire as you wind it, this can be the difference between frustration and success.

2. Heat resistance - The more heat the wire can take, the hotter your motor can run without burning up. I have seen a motor burn out (ie burn to an open circuit) a parallel strand and keep running with high temperature insulation  Mind you, high temperature wire can be difficult to solder. Often it is considered "non-solderable" which means you must burn/scrape/sand the insulation off before you can solder it.

Double coated wire
In some cases, wire may have a double coating of insulation. The outer coating is typically a slick, high abrasion resistant coating such as polyamide (aka nylon) to help in winding. The inner coating is typically the heat resistant coating. Sometimes the coatings are mixed for chemical resistivity as well.

Common insulation types
Enamel (Formvar): Low temperature (105 degrees C - class A) and poor abrasion resistance, fair chemical resistance. Cheap stuff, but it works fine for winding coils over plastic.

Mica: Low temperature (130 C - Class B) excellent abrasion resistance. Not seen very often anymore.

Polyamide (aka nylon): Low temperature (typically less than 155 C - class F) Good abrasion resistance. Very common in motor wire. Often used as an overcoat for it's abrasion resistance and it's low friction properties.

Polyester - High temperature (often 155-200 - Class F), poor abrasion resistance. Sometimes non solerable and coating should be removed chemically or by burning/cleaning. Often combined with polyamide.

Polyuretheane: Mild temperature (155-180 C - Class F & H). Often has nylon as an abrasion coating. Fair abrasion qualities.

ML - Polyimde (not to be confused with polyamide) - Extremely high temperature (up to 240 - C class H), excellent chemical resistivity, fair abrasion resistance. Often combined with nylon for abrasion resistance (polyamideimide).

Polyamiedimide - Very High temperature (200-240 C - classes F & H), High abrasion resistance. Used in non solderable wire, coating must be abraded off due to chemical resistance.

A few examples of wire
Radio Shack: Formvar enamel (105 degrees C)
GoBrushless Newbie: DBHPN - Double nylon (155 degrees C)
Micro Dan: Polyester/Nylon (180 degrees C)
Belden CDT - Polyurethane/nylon (155 degrees C)
Dearborn CDT - polyester/polyamideimide (200 degrees C)

So next time you it comes time to get wire, perhaps this will give a little assistance to find the best wire for your motor.

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