The need to step outside my comfort zone. To this day I still get freaked out by the triathlon swim start.
I like triathlon due to the fact the you get to train with the best in each of the three disciplines. Then go out and incorporate that in one multi sport event.
Making a wrong turn during the bike leg of a Half IM. I know that does not sound all that bad, but the kicker is that the pace vehicle was right in front of me when I made the wrong turn. Same thing during IM Wisconsin in 05. After I finished my second loop I missed the turn which would have taken me to the finish line. I ended up running a 36 mile marathon that day. Geez.
I love the sweet exhaustion that follows a really tough race.
Have a wicked training ethic. Be strategic and tactical and execute your plan on race day.