Role difficulty
Opinion varies as to which class is the hardest and which is the easiest. Most agree that the Valkyrie and Barbarian are the easiest classes, possibly followed by Samurai. Some say Wizards are easy, due to their good starting equipment, but others find them tough, due to their bad starting attributes. For this reason, Wizards are often start scummed.
Most find the Priest's innate ability to detect the BUC status of an item a tremendous advantage. Most find the Knight's code of conduct cumbersome, though the penalty for breaking it (1 point of alignment) is minor, unless you are roleplaying or playing a pacifist.
Cavemen can be considered a more difficult version of the Barbarian. Rangers and Rogues are also often placed toward the harder end of the scale.
Monks are a unique class whose early game is relatively easy compared to its tricky midgame, mostly due to Master Kaen and armor and weapon penalties.
Healers and Tourists have a reputation for being extremely difficult, due to weak starting attributes and poor equipment. Archaeologists are almost as difficult but have the advantage over Healers and Tourists of starting with speed and stealth.