O.K., Mike. There are many subtle differences, but I'll try to summarize some of the main points for a starter.
There are the obvious differences that anyone would notice upon stepping off the plane, like the different type sockets and the "upside down" light switches.
You'll find plenty of differences in terminology as well, like ground vs. earth, receptacle vs. socket and so on. You shouldn't have too much trouble adapting to the names, but there are one or two terms that might catch you out if you aren't paying attention closely enough. For example, be careful not to confuse a grounding conductor (an earth, or CPC if you prefer) and a grounded conductor (a neutral).
Cable sizes are measured using AWG (American Wire Gauge). It works in a similar way to the old British SWG in that the larger the number the smaller the wire, although there's no direct correlation between AWG and SWG. #14 is a common one for many domestic circuits, and is the nearest equivalent to our 2.5 sq mm.
And as I'm sure you will have noticed from other posts, the color (colour?) codes are somewhat different.
You mentioned the lower voltage/higher currents. Although 120V is the standard for domestic lights, portable appliances, etc., I've found that many people in this country don't realize that the majority of U.S. homes do actually have 240V available as well.
Whereas most homes in the U.K. are tapped 2-wires from a 3-ph network, the majority of U.S. homes are fed from a single-phase xfmr with a center-tapped secondary using a 3-wire system. A double-pole breaker is used to provide a 240V circuit to large appliances, such as a range (cooker), clothes dryer, central A/C unit, etc.
Domestic circuit arrangements use a dedicated branch circuit for each major appliance, including some 120V appliances such as a washing machine, dishwasher, and so on.
Homes will also normally have at least two "small appliance" circuits (120V 20A) feeding the kitchen/dining area receptacles. Other receptacles in the house and lights are fed on general-purpose 15 or 20A circuits based on a specified wattage per square foot.
Note that they don't use rings, and that on the general circuits lights and wall sockets can be mixed.
There are more recent requirements, such as provision of a 20A bathroom circuit to allow for hairdryers. (Contrast this with our Regs. which prohibit the installation of such an outlet!)
The main house earth is always bonded to the neutral at the main panel, so their earthing system most closely resembles our PME arrangement (TN-C-S in modern parlance), although there are differences.
When it comes to commercial/industrial, America has a greater variety of supply systems than we do. 3-phase is supplied at 120/208V Wye and 277/480V Wye, but they use delta systems as well, including an unusual (from our point of view) unbalanced delta.
The NEC (National Electrical Code) is the nearest equivalent to the IEE Wiring Regs., and is updated every 3 years. The other guys on here are much better qualified than I am to go into the intricacies of the NEC, however!
That's just a brief start to the differences, but probably enough for one message.
[This message has been edited by pauluk (edited 05-21-2002).]