I do not like big dungeons. To be honest, that is why BG2 sadly put me off a bit (really sadly because it was such a great game otherwise)
BUT I can understand gamers (especially old school gamers) to want one, as it is part of the genre
So having one big mega dungeon, with traps, tunnels, corridors, doors, monsters, treasures, the kind of dungeon that goes ever deeper and ever harder, why not. Maybe it could be optionnal, i.e. there is a way to avoid it althoug either it adds some good flavor to the storyline, or it is actually related to a big juicy side quest (the kind of quest that are close to being as memorable as the main one).