Nothing at all.
This was my third Kadett in a row. Previously I've had a GT 1.8S -87 and after that a 1.6i Caravan -88. I have been extremely satisfied with all three. Only at one occasion one of them has broken down. It was due to a broken fuel pump. Below I list my positive and negatives regarding the current Kadett:
Positive:
Extremely cheap to buy (I paid 300 sek (roughly 30 pounds) ). Even though this price was low even for a kadett they are in general very cheap to buy.
Low fuel costs (Approx 6 litres/100 km).
Easy to work on yourself.
Cheap, and easy to come by, spare parts.
Good sized trunk.
Handles very well, even at high speed (well, high for a kadett).
Extremely reliable. Starts regardless of temperature.
Comfortable and fun to drive.
Good heating in the winter.
Negatives:
RUST! If Opel had spent a few extra D-mark on rust protection the Kadett would have been one of the best cars ever made (My humble opinion of course)
"Squeaking". Sometimes when going over a bump it sounds like the whole car is coming apart.
Freezing locks in the winter, a problem not exclusive for Opel, but still very annoying.
I own an Astra 1.3 gl I also think its one of the best cars made in the 1980's and maybe even the 90's I want to buy a GTE/GSI.
I drive a 1982 Kadett 1.6 GLS (from the D-range of Kadetts). The car is a hand-me-down from my mother, but the thing still motors quite well! (Well, for a non-fuel-injected car that is) I have to agree on the rust-problem though, the bodywork will be gone long before the tough little engine finally decides to retire!