Ljgj brings up a good point that I have thought about quite a bit: "we worry about the speaker cables but not much is said about all the wire used inside the equipment before it".
Indeed, the "speaker wire" in most speakers (most have wire from binding posts to crossover, then crossover to drivers), is not very impressive. However, I am of the opinion that a high end speaker is a complete and fine tuned system, where the designer/design has accounted for every piece inside. It is now a "closed system", so whether it uses silver or copper wires for example, is almost irrelevant because the end result has accounted for it. On the other hand, the cables we are talking about here, are cables that are not only "outside" of the speaker system, but also connecting another "closed system" on the other end - the amp. So the cables I buy are more a matter of _mating_ the two closed systems, to produce a result that is pleasing to me. Thus I find that "good" speaker cables improve or hurt the sound of a system, almost regardless of the quality of the parts inside the speaker or amp.
Also, I do agree with some of the others' comments about interconnects making a bigger difference in general than speaker cable.
In answer to the original question then, the audible differences in speaker cable I have found are:
EVERYTHING.
Bass - it's speed, attack, decay, quantity
Mids - clarity, sharpness, space, presence
Highs- crispness, space, air, sibilence.
All of these have been effected in my playing with speaker wires. So, I think the question is more of how to define the differences, than a question of "if and what".