ETOPS requires specific maintenance procedures, and certain levels of equipment redundancy. I don't know what 737 NG's come with as I never flew them. I know lUAL's ETOPS 757's had some extra stuff over the non-ETOPS ones. If I remember right a HMG (hydraulic motor generator) and an extra HF radio if I remember right. I haven't flow 756 in about 8 years so my memory isn't so stinking good anymore.
No big deal really from a pilots' perspective.

The system differences are harder to remember, especially equipment cooling. I think I could fully explain all 756 equipment cooling for about 1 hour in my life. Luckily that was the one hour of the oral exam of my first 756 transition course (I did it 3 times).

"It is not what you know, it is when you know it"