Probably the low down torque, and the high mileage many of them have done. Lots of stop start, particularly the starts.I own a 1.4i petrol, and although I don`t have as much get up and go as a diesel, petrols are all blessed with a lovely gearbox, it`s currently having it`s cylinder head re furbished, due to valve problems but thats another story.
From what I`ve read Diesels do seem to have more gearbox problems.
Good Luck with the car.