There seems to be a line in the sand at which DLC passes from acceptable to unacceptable. If DLC resembles something like "The Passing" in L4D2, or any one of a number of updates to TF2 (I know, both valve examples, work with me here people!), where more content is added for free, or if the content that has to be purchased doesn't affect the core gameplay in any fashion, then it's acceptable. I totally agree with this. I usually only have money for 2-3 games a year, being a university student and having to put a roof over my head and food in my fridge. If those games are stripped of content that WOULD have appeared had DLC not been invented and instead I am forced to make a multitude of micro transactions to get them all, I am going to be seriously cheesed off!
So, if publishers and devs want to charge non-essential DLC (Like Horse Armour) that just adds shiny things and doesn't affect gameplay balance, go for it. HOWEVER, woe be to the people that decide to cut a crucial plot point and charge me extra for it down the line.
As for the issue of DLC vs. Expansion Packs, the whole point of an ep was to add a shit-ton of content to the game. Take for example Red Alert 2 (yeah, it's going back a ways): The initial game was about 20 hours long, if you went through both campaigns and had some fun in the skirmish modes. With the expansion pack of Yuri's Revenge, you got another almost 20 hours of gameplay, a whole new faction, and a non-essential plot development. The same thing with Mechwarrior and the Mercenaries expansions. More mechs, more maps, and a completely unrelated campaign that didn't affect core gameplay. Perfect.