The face of the killer
To be honest, half of me just thinks they never should have revealed the face of the "hitchhiker." Plus, I agree with everybody that it was a potential "High Tension" ending. When they revealed the killer as some rabid, JRARRAGghghHHH, maniac, it just got zany. I thought the subtle faceless killer was much more mysterious and creepy.
But hey, that's just me.