I think those of us that have seen the show a number of times enjoy the little subtle things that happen during the performance that you don't notice the first time you see the show. The more you see the show the more you notice how these things enhance the production. Some of my favorites are:
Frankie's facial expression when Joey mentions Bob Gaudio wrote Short Shorts.
Mary smoking Frankie's cigarette when Francine calls after she tells him not to light it because she just got out of the hospital.
Watching Frankie getting angrier & angrier during the table fight until he yells at Tommy.
The drummer leaves the stage after the audience leaves when they are singing "I Go Ape".
Frankie flirting with Mary sitting at the bar while singing Sunday Kind of Love.
What are some of your favorite subtle things?