Based on a list I made and kept as a poster over the crib:
Hungry/Thirsty
Diaper wet/dirty/tight
Unswaddled newborn
Teething
Gas pain
Reflux
Too cold/hot
Lonely
Bored
Tired
Rash
Sick
Hurt self
Fear of dark
Too much light/noise during sleep
Startled
Strangeness in environment
Missing toy or other object
Foreign object on skin
Hair-snared digits