By far and away the most annoying petrol station trait is when people don't advance to the furthest pump. The station I use for work can fit 3 cars lengthways but some bell end in his/hers precious car will stop at the middle pump leaving the first one completely redundant. Nobs.
Something I really hate is where you have a station on two roads, and people going through it both ways. I've been blocked in twice at the Shell garage on Preston Road/London Road, by people coming in opposite directions. They did put signs up but they were ignored and then they removed them again, You even get people going through the Esso garage on London Road in the wrong direction quite frequently. How hard is it to understand?