This is Reynad's Warlock Aggro Deck, that he named Zoo.
Reynad praises this deck as the best deck in the game, despite the fact that it can be built on a budget. I have included a link to his video where he goes into a lot of detail as to why he picks each card.
I had already felt the Young Priestess was under performing. Adding the Abusive Sergeant gives you more burst potential, which is nice. The Argent Commander drop was a little more surprising to me, but after trying the switch I definitely like the change. I feel Dark Iron Dwarf fits into the curve better due to coming before , thus reducing the chances of having to discard a high priority card. Dark Iron Dwarf is durable at 4/4 and adds the same burst that Abusive Sergeant gives. However I am not sure that Leeroy Jenkins and one Dark Iron Dwarf is not the better choice.