Using Borderlands, Fallout 3, Dragon Age: Origins and Mass Effect 2 all as good and bad examples of how to do DLC right, IGN AU's Scott Donaldson tries to explain why the downloadable content model is actually good for the video game's industry in a fairly brief but not light on content editorial. Here's a sampling:
The fact is, whether or not DLC is having a positive impact on the gaming community shouldn't be determined by cost, because only rarely does a content package need to be purchased in order to get the most out of a game. And nobody knows this better than the gamers who don't purchase DLC at all. If premium content were to become an integral part of gaming rather than an optional one, it's these gamers that would suffer.
As such, the success of DLC should be measured in its ability to provide entertaining content on top of games that are already complete packages, and this cannot be demonstrated better than by Gearbox's first person loot-'em-up, Borderlands. Its four campaign add-ons are shining examples of how a developer can use DLC to build upon an existing game without making gamers who choose not to purchase extras feel ripped off.
Borderlands is a big game to say the least. A massive campaign coupled with countless side-missions and a huge incentive for extra play-throughs means that players are faced with a staggering amount of stuff to keep them entertained in the long run. Even if there wasn't any DLC to buy, we would hardly feel ripped off by its length. Furthermore, the main story is totally done and dusted by the time the credits roll. As such, players don't need to purchase the DLC in order to get the most out of the game, because it's all there to begin with. So the fact that Gearbox developed three huge content packages and one incredibly addictive small one and released them for about ten dollars each is fantastic. Are they entertaining enough to justify their cost? Absolutely. Are they crucial to our understanding of the main story? Not even slightly, and that's the clincher.