Don’t get me wrong. I really like DLC when it's done right, and it's something that I think is going to become a very vital part of the industry, if it hasn't already. (And really, I think it already has.)
That said, certain companies and their practices are really starting to piss me off.
In my mind, it seems to me that the proper role for DLC is to extend the life of a game after it's been out for a while and would ordinarily have been put aside in favor of newer releases. Perfect examples would be something like the add-on missions for Oblivion, Mass Effect or Fallout 3, each new piece of content able to reignite interest in games that would likely have been traded in or covered with dust on a shelf if not for the knowledge that something else would be coming down the pike. A trait common to all of these is that their core games were all unquestionably complete in and of themselves, including all the trappings we'd expect.