noobie alert: how does a cable make cans sound better?

Well, the items you mentioned are all important, and of course to varying degrees may be difficult enough to do on their own -- hence using different materials to conduct better, different shielding to prevent interference, different designs to reduce capacitance (though as I understand it that's rarely an issue with most interconnects due to their length if nothing else... forgive me, I'm not an electrical engineer). However, people have argued long and hard about other effects as well, particularly second- or third-order effects. Many of these are more difficult to measure and most of what I've read seems to be more- or less-educated guessing and theorizing, rather than anything solidly, definitively proven. Mostly, people seem to claim that certain cables sound better to them, whatever the actual causes.

Anyway, here's some effects you didn't mention. Certain types of electrical signals (for example, at different frequencies) might propagate faster or slower through the same material. So you can lose coherency as the complete waveform moves down the cable, leading to parts of the wave being out of phase, etc. This can muddy the sound. Some cables may not pass certain frequencies well at all, leading to a filter effect (like a high-pass filter, for example, if the cable is less transmissive of bass signals). Etc. Do I know how and why any of these happen? No, not really. However, given others' reactions, I'm at least willing to suggest that different cables can make things sound differently, even if I'm not convinced that there's a reason for a $2500 interconnect