I mind my own business, unless the actions are imposing externalities on me. I can't care less whether someone doesn't buckle his/her seatbelt or doesn't put up his/her tray table, unless there's a direct effect on me. I might say something if the noise level doesn't remain acceptable, otherwise, I leave the policing of rules to the crew.

Somewhat related story - on a recent flight from ATL to CHS, some guy in the aisle seat adjacent to me seemed very proud of himself for tattling on somebody who didn't turn her cell phone off before the flight. The lady wasn't actually ON the phone, but I guess the guy could see that it wasn't off, so he stopped a FA, asked "aren't cell phones supposed to be off now?", got an affirmative answer, and then proceeded to gleefully point out the woman a few rows ahead of him. All I could think was, "what a jerk. I'll bet you were the guy ratting out your friends in high school who were making fun of the teacher behind his/her back."