In another case of embarrassing ignorance, it took a couple of years for me to discover that you could eat food and drink drinks at the same time. Though I've played since open beta, it wasn't until the tooltip started showing up on loading screens that I found that out. Yay tooltips!
What about you? Did you know about BoA items being cross-faction, but not cross-server? Is there anything in WoW that you've discovered over your playtime that made you facepalm? Fess up!