Hello there.
Over the years Blizzard added more and more things to the game, no matter how big or small they were. So, what is for you the best addition to the game? Heroic Dungeons? Pet Battles? Achievements? Maybe something small, like a specific questline or a certain item?
For me, the best addition came with Cata: Interesting questing/solo encounters. Before Cataclysm, each named mob and quest boss had the exact same HP as any other mob of the same level. Most of the time they also had no interesting mechanics. Only Dungeon and group mobs were somewhat interesting, but solo mobs? Nothing special there. Actually, only 2 times I thought that they made an interesting solo encounter. The first was Shartuul's Transporter in BC, the second time was the final battle with Overlord Drakuru in Zul'drak. All other named mobs were extremely boring gameplaywise, because they were as strong as any other mob in the area.
And then, enter Cataclysm. When I saw the battle against Sethria for the very first time in a beta video I completely geeked out. That is a solo mob that has a huge health pool. He is able to use several abilities. Heck, he even had a second phase! And as the game progressed, it only got better. It might be a really small addition for many people, especially compared to some of the huge things, but for me it's one of the best things Blizzard ever added to the game. And I highly doubt things like the Brawler's Guild or even the current rare mobs would exist without making solo bosses more interesting in the first place. This also strengthens the lore, because important characters fought during a solo quest are now actually stronger than the their minions.