This past April, I had been playing World of Warcraft for one and a half years. I had all the latest add-ons, knew the math behind all the stats for my characters, participated in plenty of online discussions, was an officer in a BWL guild and was basically a WoW dork to the extreme. Then on one April afternoon I was playing WoW at my friends house and went to loot a corpse. I right clicked on the corpse, the window popped up, and I clicked on the silver and cloth, then moved onto the next mob.
My friend looked at me like I was a moron, and asked..."Um, don't you shift-click?". I said no. He laughed. Then he told me that when you loot a mob, if you hold down the shift key, you will auto-loot and save yourself a little time. And I have been shift-clicking ever since. And to this day I don't know how I played for 18 months with that little bit of information alluding me.
What about you guys? Anything in World of Warcraft that you discovered way, way after you should have?