I think that there's a very valid pattern of complaints over DLC.
Not all of it is portable between different machines (the new Halo 2 maps are an example). While, from a strict licensing standpoint, you're not supposed to share your game content with your friends, I believe everyone has let someone borrow a game or taken a game over to someone's house. Portability and borrowing game content is strictly limited.
None of it has any resale value at all. If you buy a disc-based expansion pack for a game, that disc has resale value, whether to a private consumer looking to buy your games or to a GameStop/EB. You cannot sell DLC at GameStop.
Durability of individual HDD/memory card content is not as reliable, and companies may not host that content forever. What if you buy an extra CoD2 map, and then give your Xbox 360 to a cousin five years from now and the HDD fails. If the map is no longer hosted for download (given that you've already paid for it), how can you retrieve the data? Doesn't work that way for a disc.
It's not that DLC is valueless. I've purchased my fair share when I thought it was justified. But $2 a song is significantly overpriced to me when it's over a DLC medium. I wouldn't pay $2 a song if I were buying it on a disc - why should I pay more for a worse delivery format?