DLC itself isn't a bad thing or a good thing. Its a tool, something that used to be called "expansion packs". Despite what nostalgic views people have, not all expansion packs were great, not all of them were huge additions to the game, and some of them were absurdly priced... almost the price of a full game.
That being said there are some absolutely stupid DLC practices (SFIV and Mass Effect 3's on disc I'm looking at you) and those should definitely not happen if one wants to keep consumers buying one's product.
I'm also one to say that DLC is not a right because you bought a game. Meaning that not having said DLC does not equate to the game being somehow incomplete when released (though that can happen and companies that didn't finish certain parts pre-release should release them... Ubisoft ACII, now I'm looking at you).
Not all the cool shit developers want to put in games can make it into the game, and a developer/publisher is not required to put those things into the game post-release (they also shouldn't advertise features or content that didn't make it either but thats a different point). If said developer/publisher chooses to work beyond the point where the game build is in its release-final form to add extra content to the game in the form of DLC, that is their prerogative. How they go about releasing said content and whether or not they attach a pricetag to said DLC is up to them. However they should take care if they're to price said DLC to do it in a way that makes customers feel they've got their money's worth.
Also DLC should never make a player feel he/she is missing a vital chunk of the game without the DLC, unless we're talking a huge expansion pack or something.
Once a game is released, unless it is completely broken and unplayable, it is a finished product. Anything released afterward, aside from bug fixing patches, is nothing the dev/pub owes the customer.
If they choose to release DLC for free, thats great. If they overcharge for something clearly not worth the asking price, don't fucking buy it. Demanding they release all post-release content for free because "I bought the game, you owe me" is bullshit. They owe you nothing except a game that is playable all the way through and patches to fix any issues that would present said thing from happening.