1. People who fail to comprehend the difference between the Front Row Queue and Regular Queue at the ride station.
They run down the front row line thinking it's shorter, realize it's not, then try and push there way back through to their previous position in the regular queue. All whilst in the cramped, crowded space of the station; holding everyone up. Just read the sign first!
2. Loud music everywhere. (I'm mainly thinking of Thorpe with this one) but even at Oakwood, I couldn't sit down to have my lunch without a speaker nearby blaring out music at an invasive level. Parks need more quiet, chilled spaces for when you want to take a break and relax.
3. Unclear park maps. The ones which are just vague drawings with a key down the side, and don't actually show where each ride/show/food place is - just the general area. When visiting a park for the first time, you want to be able to get as much out of the day as possible; not spend it wondering around trying to locate things.