The quests were literally awesome, really funny and really intriguing.
The quests of Thieves Guild, as well as the dailies the guild provides, is the main reason of a thief considering buying the Thieves Guild DLC. Well if you ask me, it does! Not only for the passive skills that give you, I would say that this is the least actually. Though being a master thief is demanding on skill points and gets some time to get used to the techniques.
Stealing is the easy way of making money, and in my opinion at least, is actually pretty fun as well. Even though trading can sometimes be more profitable, it takes some hell of a boring time in comparing prices and finding the proper traders for the right items. Becoming a master thief in ESO is, in my opinion, the best way to earn money if you are not really the type of player that will consume time in guild trading to earn some coins.