Since I "went kosher" in June 2010, the most annoying thing remains drinks. I am apparently incapable of remembering that drinks can and will be dairy.
I don't drink milk and I rarely eat meat products, so my kitchen is almost entirely dairy. I rarely have to think about dairy v. meat except for what I bring into the house. In those rare times that I'm fleishig (aka, in the period after eating meat when dairy is prohibited - thankfully, only 3 hours for me), it's always drinks that trip me up. I can remember to avoid eating cheese or yogurt, etc, but I have the hardest time remembering that many non-dairy-looking drinks are actually dairy.
I'm much better now than I was several months ago, but only because I always remember that this is a mistake I'm susceptible to! As a practical matter, this means I try to avoid eating or drinking anything but water in those 3 hours. This would be much more difficult if my minhag were 6 hours!
(As this is a post I thought of a few months ago, it hasn't been a problem in a while now, yay!)