What do I hate most?
Small detail parts with such huge sprue attachment points that no matter how hard you try, the part breaks apart before they seperate from the sprue!
Long thin pieces like the Revell/Germany M2A2 bradley's Bushmaster cannon attached to the sprue at both ends instead of at one end, so it snaps in half when you try to remove it from the sprue!
Unclear instruction sheets.
Directions that ask you to mix %'s of different colors to get another color.
Those last two were mentioned I believe. Still, the top two are my pet peeves, particularly when building Ships and Armor.