DLC has its good and bad points
ideally it's a great way for developers to give additional content that they otherwise would have been forced to completely cut out of the game due to deadlines
in reality it's all too often a way for publishers to make additional money by cutting bits out of the finished game and selling them separately afterwards
there's also a rather vile tactic where they give free DLC to people who buy the game brand new and sell that DLC for a price high enough to make buying the game second hand more expensive than brand new when combined with the DLC. a rather stupid tactic too since it's more likely to encourage piracy than sales
Witcher 2 is definitely a great example of how DLC should be handled. WoW too, look at all the content patches we've had over the last six or seven years and they've all been free (expansion packs excluded of course). granted we pay a monthly subscription, but does anyone honestly believe EA won't be trying to push out paid DLC for Old Republic?
personal experience I've found very few DLC actually worth paying for. there's a few exceptions like Lair Of The Shadow Broker, Bring Down The Sky and Soldiers Peak (although I got two of those for free) and some are obviously not worth paying for (entirely my own opinion of course) like Horse Armour or Weapon Pack 1443435 (where gamebreakingly powerful weapons appear in your inventory with no effort at all) and some look like they might be good but turn out not to be
I also dislike the practice of having DLC continue the story and essentially making it mandatory. DLC should be additional and optional content, either telling side-stories or giving additional information. Think of DLC like the extra features on a DVD movie release. you wouldn't expect to have to buy the DVD for the ending of the movie, but you might get some deleted scenes
DLC like Arrival (Mass Effect 2) and Witch Hunt (Origins) which continue the main story should, in my opinion, be free if they're not part of the main game