Recently, I found that old thread by C-H about Why cables look like they do and revived it with some additional information. This thread is for those other questionable norms in the electrical world. I'll start with...

Voltages
I think it's been established that they don't save much of anything on insulation by going with 120V instead of 220/230/240V for small appliances in North America, as almost all mains cordage worldwide is good for at least 300V* operation. I've seen it suggested before on this forum that they add 240V outlets in kitchens in new buildings there, doubling the available power for kettles etc. That idea seems sensible to me. Implementation would be straightforward enough - Leviton even makes a combination NEMA 5/6 outlet, as shown here (a 15A version is also made), although it's not available with shutters (or "tamper resistance" as they like to call it).

As I understand it, the only real advantages of the lower voltage are:
  • Works better with incandescent lamps (thicker filament = longer life at the same temperature, or higher temp. and by extension efficacy with the same lifespan) - but that's less and less relevant with the advent of LED lighting
  • Easier winding of small motors and transformers (half as many turns of wire with double the cross-sectional area)
  • Shorter creepage distances on circuit boards (although that only matters if a board is designed exclusively for the lower voltage)
I don't suppose the somewhat weaker (though still quite lethal) shocks are enough to make up for the lack of protection NEMA plugs provide from contact with the pins.

*Well, AS/NZS 3191 specifies 250V for light-duty and 250/440V (the 440V being between lines in a multi-phase circuit) for ordinary-duty cords, but I believe that's merely a political difference, as the dimensions are unchanged from the harmonised cords which are rated 300V for light-duty and 300/500V for ordinary-duty. The heavy-duty cords, curiously, were up-rated from 450/750V [harmonised] to 600/1000V [AS/NZS]. There's also XTW for decorative lights in North America - I happen to have a "plug-pack" supply (a linear type providing 6VDC 2.1A, unregulated [measures 9.6V unloaded]) using a 20AWG parallel version as its output lead, and it's marked with 125V. But XTW is the exception to the rule.

Polarisation
Let's just say that too much attention is paid to it in North America; not many appliances care much about it. Well, with BS 1363 correct wiring of the outlet is critical to ensure that a short to earth doesn't bypass the plug fuse, but correct line/neutral connections in the plug are no more important than with any of the others. Don't want something (e.g. a toaster element, or an incandescent lamp socket) to remain live with the switch off? Equip that appliance with a double-pole switch. They don't add that much to the cost.

Speaking of toasters, I can't say I follow the arguments given for not earthing the North American versions. If the idea is so the user isn't electrocuted if (s)he contacts the element with a knife/fork in one hand while holding the case in the other, then all you need to defeat the argument is a stainless steel kettle - or a kitchen sink with metallic plumbing. I suspect that the real reason is economics as usual, and that the UL is a corrupt organisation; I still don't get why else you'd bother with separate "residential" and "commercial" fittings. I guess this also explains why they don't use the double insulation symbol (except on power tools?). (Any info on toasters in Japan?)

And why the heck don't the Japanese use more of the proper 3-pin outlets (which they do make some use of) instead of those kludges with the earth post near 2-pin outlets (to which the user is supposed to manually attach an earthing fly-lead with a lug on its end)? rolleyes