Electricity

The Brits have funny electricity. (why should that be different?)
Electricity is at 240 volts AC, 50 hertz. US power is 110 volts at 60 hertz (aka cycles).
1) Obviously, everything needs new plugs. Big, bulky plugs.
2) Because of the reduction in cycles, US electrical things run slower here so clocks and timers don't work properly. Other products don't get enough power to work well.
3) Step down transformers work with some things not others, we decided not to risk bringing anything electrical. Our friends, the Zanettes, brought their TV and it was ruined the first week.

However, replacing things can be expensive. I use a Sonicare toothbrush which I left at home. To replace it would cost £100 ($200). I paid less than $100 in the states for ours.