The main difference between ETOPs and non-ETOPS 757's is the former have an hydraulic driven generator which can be used as a fourth AC electrical source. This powers several busses in the event of a double AC bus failure and provides amongst other things the captains EFIS and instruments, left FMC, left HF and so on. In the non-ETOPs it can get a bit dark and basic on the flight deck with a double AC bus failure with the additional pressure of flight beyond 90 mins resulting in loss of electrical power.