Hi Bob, I'm a little late to the party here - been traveling the past couple weeks.

Anyway, the simplest answer is that the wrapper is the nicest looking leaf - smooth, unveiny, no holes or discolorations or other imperfections, etc. It also tends to be thinner but doesn't have to be. Certain strains of tobacco make for better wrapper leaves and are grown for that purpose. Wrappers tend to be the most expensive for obvious reasons. Often you'll see the wholesalers selling a type of tobacco as wrapper and also as filler for a lower price - the wrappers have been selected out, and the fillers don't need to be as nice.

Binders are a different animal - a good binder is generally thicker and tougher as it does most of the work. It's not really interchangeable with wrapper. Binder leaves should be free of holes for proper burning, and shouldn't have too much relief to them as the wrapper needs a smooth surface to cling to.

Filler leaves can be just about anything, and can have imperfections, discolorations, etc, but nothing so bad as to affect burn or construction.

Regarding the flavor of the wrapper, it isn't a one-dimensional answer. Flavor comes from the whole blend, of which the wrapper is a certain percentage, depending on ring gauge and # of wrapper layers. Certain wrappers such as maduro have distinct flavors, but again, it's just part of the whole blend.