this is a subject Maelfyn and i have discussed on the boards a few times since i started playing. its an important topic though, and i'd like others to chime in with their experiences.
basically just say what level / class you were when you beat XXX boss on XXX difficulty, and say whether or not you thought it was too easy, too diffuclt, just right, etc.
recently i beat Vox with my level 67 SK on Nightmare and it felt way too easy. finished the fight at full health and 80% mana.
just now though I beat Nagafen at level 70 (same char, SK), and finished the fight on my first try but with about 15% health and pretty low mana. difficulty there felt a bit more appropriate. was a super close fight, i was a few seconds from dying. personally i wouldn't have minded if it was a bit harder, but i REALLY like a challenge and idk if everyone else would agree.