It is readily agreed that the Dark Brotherhood questlines are amongst the finest that The Elder Scrolls has to offer. They often put aside the high-fantasy elements of the world, cut the proverbial crap, and indulge in simple whimsical quests that are executed to resounding success. Instead of pitting you against an evil demon, you have to go kill go kill a bride at her wedding or a mad homeless man.
They are really the perfect example of simplistic game design. They often simply give you a simple objective that under any other circumstances would be boring, and let you tackle it how you want to. More often than not, they represent the more free and immersive quests in the games.
So, it is no wonder that these specific quest-lines are what linger in people's minds long after the games are over. In a world that is incredibly saturated with the same fantasy nonsense, it is incredibly refreshing for a game to indulge base desires and out-there ideas. They really are a testament to good quality quest design, and hopefully every quest in Elder Scrolls VI will have the same polish.