DLC is, generally speaking, really quite terrible. Short, over priced snippets of content that were clearly afterthoughts.
Bethesda have the best - and worst - DLC on the market. Some of their stuff is, frankly, horrendous. Other ones, though, are the reason why DLC could be quite good.
The problem is two fold:
1. DLC is seen as a way to make post-release money, as opposed to being a way to extend something you like.
Publishers, who fund the creation of the DLC, aren't going to foot the bill for a lot of additional content that - at most - 51% of people are going to actually buy. So DLC contains a lot of recycled everything. This makes it crap. If it were cheap, this wouldn't be so bad. But, Publishers don't understand that charging US$15.00 for a 30 minute quest that rates 1/5 stars isn't ok. Why should they - Activision Blizzard charge US$15.00 for their Call of Duty Map packs that contain mostly recycled maps from previous games, and they make a fucking fortune.
2. DLC is used to circumvent your right of purchase.
DLC is used to make sure you make as little money back from your purchase as humanly possible. If you sell a game you have DLC for, they've been paid and the amount of money you can possibly recoup is dwindled. They, however, don't tell anyone that anyone can buy DLC for a used copy - leading to literally thousands of purchases for DLC that enable them to recoup quite a lot of money.
The funding for the DLC is pre-approved. When the game is put into development, the overall budget is split between the base game, and the DLC. This means when you buy the game, you've paid for the development of the DLC. The more DLC a game has, the less content exists within the retail purchase.
DLC is like 3D Movies: a terrific idea was milked to death before it could even crawl out of it's mother's vagina and take a fucking breath.