How is "information relating to the safety of flight" defined by the FAA? 14 CFR 91.183 states:


  Unless otherwise authorized by ATC, the pilot in command of each aircraft operated under IFR must report the following as soon as possible: Any unforecast weather conditions encountered; and any other information relating to the safety of flight.


This language is very vague and probably causes a lot of confusion for pilots who are studying it. Does the FAA elaborate on this concept or provide an examples or a more specific definition somewhere else? If not, what are some examples of reports ATC would expect based on this reg?
The language is intentionally vague.
Much like Justice Stewart said about pornography, when it comes to "information relating to the safety of flight" you'll know it when you see it. 


Birds flying around in the approach path? Yup.  
Wind shear on short final? Yup.
(Even if it's forecast/reported telling them how much airspeed you gained or lost helps.)
"Drone" buzzing around in the traffic pattern? Yup.
Deer grazing in the middle of a grass runway? Yup.
Bird strike? Absolutely.
(There are regs/procedures specifically for this.)
The guy in row 3J is threatening the flight attendants? Yup.
The guy in row 3J spilled his soda on his lap? …Not so much.