Role difficulty
Role difficulty is the general difficulty of being able to successfully ascend with the various roles.
Opinion varies as to which roles are the hardest and easiest. Most agree that valkyries and barbarians are the easiest classes, possibly followed by samurai.
Wizards are usually easy due to their good starting equipment (for this reason, wizards are often start scummed). Those that attempt to play them as physical combat characters will, however, find them difficult due to their bad starting attributes.
The priest's innate ability to detect the BUC status of an item can be a tremendous advantage.
The knight's codes of conduct can be cumbersome, though the penalty for breaking them (1 point of alignment) is minor, unless you are roleplaying or playing a pacifist.
Rangers and rogues can either be difficult or easy, depending on how capable one is of making use of their powerful ranged weapons (especially in the early game).
Cavemen can be considered a more difficult version of barbarians.
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. Archeologists are almost as difficult but have the advantage over Healers and Tourists of starting with speed and stealth.