There's some things that really annoyed me in Lost, such as the fact that BiB didn't know what "dead" was, but then later said that the hunters killed a boar. Or the fact that we never found out what the Numbers meant exactly.
But there is one thing that annoys me more than these, and it's how much the characters faces changed, it's unbelievable... Probably the three most drastic changes are probably Claire, Jin and Sun. Let's see:
Sun's face change is probably the most annoying to me :P