The thing with DLC is a lot of the time they were forced to leave it due to time or space on the disc. Also, a lot of DLC is made well after the game hits store shelves and the creators get feedback on what would be cool new additions. Take the GTA4 DLC, I literally just beat The Lost and the Damned like 2 hours ago, and for a measly 15$ I got 12 hours of new story, new weapons, and new online modes. As for Borderlands DLC I've heard it has new weapons and adds to the story. Not just pointless crap. Plus it's cheap and more achievements.
I can't wait for the Assassins Creed 2 and Modern Warfare 2 DLC to come out.