Other - Poor batching.
This REALLY pisses me off, especially if there's a massive queue, and gaps are being left unnecessarily. I realise that it's maybe not as easy as it looks, and that you can potentially waste more time by waiting to fill gaps, but often there's no excuse. One train operation on Speed with 2 adjacent seats (bear in mind, that's 1/4 of the seats on the train!) going empty on basically every other circuit, causing an hour long queue. Its not as if its hard to find a 2 to fill a gap. Rides that have airgates, but don't fill them until they're open, then wonder why there's empty seats should all be burned. Having 2 single riders in separate rows without asking them to pair up and create space for a pair is also another classic example.
Equally though, it makes me really happy if I see it done well. We saw a couple of very good examples at Thorpe last weekend which really did help to improve my opinion of operations at the park.