I'm mostly looking for difficulty in boss encounters, but everything must be taken in moderation. A too difficult boss (especially if it's a MK Walker boss) just breeds frustration and practically guarantees that the game won't be touched afterwards. Ideally, a boss would test your abilities in non-obvious ways. For example, the Sword Master in Secret of Monkey Island. All of the other pirates have distinct insult-reply pairs, but defeating the Sword Master requires you to look beyond the pairs and find appropriate replies to the Sword Master's unique insults. The first duel usually ends badly and thus ramps up the Sword Master's status for the time that you actually defeat her, maintaining the narrative.
One of my favourite mixes of narrative and heroism occurs at the end of System Shock, where you finally corner the elusive Shodan into a single computer system. Just getting there required you to use your problem-solving and combat skills to the fullest, and the added control screw mechanic made Shodan the deadliest cyberspace opponent in the game. Which is appropriate, because you're fighting her on her turf. Facing her again in "real" space in System Shock 2 felt downright anticlimatic by comparison, and not just because she was a pushover. The narrative's momentum stopped at the moment you destroyed the Many's brain, getting rid of the primary antagonist. Taking care of Shodan felt like mopping off the blood after a firefight. She should have been left as an optional (and hard!) boss fight or even gotten a full-blown sequel/expansion dedicated to fighting her.
One of my favourite mixes of narrative and heroism occurs at the end of System Shock, where you finally corner the elusive Shodan into a single computer system. Just getting there required you to use your problem-solving and combat skills to the fullest, and the added control screw mechanic made Shodan the deadliest cyberspace opponent in the game. Which is appropriate, because you're fighting her on her turf. Facing her again in "real" space in System Shock 2 felt downright anticlimatic by comparison, and not just because she was a pushover. The narrative's momentum stopped at the moment you destroyed the Many's brain, getting rid of the primary antagonist. Taking care of Shodan felt like mopping off the blood after a firefight. She should have been left as an optional (and hard!) boss fight or even gotten a full-blown sequel/expansion dedicated to fighting her.