Reader Morncreek just recently informed me in the comments of the results of last week's Community Blog Topic that heirlooms were cross-faction as long as the characters are on the same server. Mind blown. This may be common knowledge, but I had absolutely no clue. You can't send cash or even just text, but you can send any Bind on Account item. In my defense, all of my Alliance characters are on different servers from my Horde characters, but still.
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!