Pretty much what the gaming industry is becoming. My personal opinion on the matter: It’s theft. If people do the effort to invest their disposable income in games, they should get what they paid for. They paid for a full game, and deserve to see all the mandatory content.
What should DLC be? It’s simple: Novelty items (Extra character skins, weapons as long as they’re not more powerful than all the other weapons in the entire game) and extra content that goes beyond the game’s main story, not a continuation of the story. That’s what sequels should be for.
The industry should keep moving forward, and DLC is a good thing for a game’s longetivity, but at least make sure it’s additional content, and not just a “key” to unlock content that’s already on the game’s disc.