Diablo III replayability was boring. The dungeon layout and critters placement was always the same (and loot had no sets or unique characteristics).
I understand that hand designed layouts may look/feel somewhat better, if so make the first time in the dungeon the hand tuned version and then randomized subsequently. At least allow some areas to be swapped and rotated. Not every boss or mini-boss monster need to behind a set of fancy doors or special entry, makes me more fearful if you are never sure what lies ahead.
And lets not have everything static, some wandering monsters or groups of monsters that we hide from or engage as we wish make the environment more dynamic. Perhaps some of the wandering monsters(Kobolds, gel cubes, or something to fearsome avoid altogether) are noisy (sing a song or instrument, growling, stomping, banging weapons on the wall, fighting other monsters or among themselves because they are bored. So we can detect them before they arrive, or they are sleeping, or eating a meal, playing cards, and so are initially stunned. Or they try to sneak up on the players (esp while engaged with other opponents). Perhaps opponents have a bell, pull cord, horn or gong that summons reinforcements unless players are able to stop the runners. And traps are always interesting. Perhaps your rouge spots them (or you find a map that marks a few) and then the trap appears for all players to avoid. The ability to push opponents or players back (into a trap, flame, over an edge). Perhaps mages can set temporary magic traps and players can lure(barbarian taunt, etc) monsters into the traps.
Do some monsters deeper in the dungeon level hear about others encounters(if runner monsters get away, tell friendly monsters, and join forces with) and so no longer fall for players old tricks.
intelligent monsters target mages and healers, sneak up behind, yell down hall to friendlies, secret passages and doors that open behind players with back stabbers, or oil lobbers. Block escape doors behind players.
A bit of dungeon ecology, a monster nursery/hive, mushroom food growing room, weapon crafting room, barracks, slavers, etc.
