So you can only have 2 copies of each individual card in your deck, so if you get a third copy of a card, you can only disenchant it ... right? Or is there anything - like, really anything - you can do with it other than disenchating it? Or can I safely disenchant every third copy of a card I get?
There is nothing you can do with a third copy. The only reason not to directly disenchant them is if you're speculating on a nerf of that particular card. When Blizzard changes a card you can exchange it for the full dust value for a short time, much more than you usually get.