I would consider saying something when the actions of others may potentially affect my safety. For example, if someone in my row or the row in front of me is hindering access to the aisle (and therefore exit) during takeoff or landing, I'll say something. This would be things like bags on the floor, tray tables down, seats reclined. I might also say something if there's a heavy object free to fly around (say, a laptop on someone's lap).

Seatbelts, cell phones, small toys, etc... none of that bothers me too much.