The problem with DLC is not the concept, but how it's implemented and what it implies. I agree with Gilrond: DownLoadable Content is as broad and ambiguous as it can be, which means that in practice anything passes as DLC. Characters, missions, weapons, even cosmetic crap like costumes and accessories! Most of these are non essential, and should have been part of the game from the beginning. The worst part is people pay real money for this garbage, and companies aren't idiots: they see a way to maximize profit by selling game content separately and they make the most of it. Take day one DLC for example, it's an insult!
Of course anything can be DLC, but most of it is garbage. As long as we keep buying horse armor and ranger modes, we keep feeding this system. Imagine if Aerondight had been a $5 DLC for TW1...
So I also dislike this acronym and believe we should call things by their name. Expansion, mission pack, module (standalone adventure), barbie doll make up kit, etc.