Nothing of paramount importance.
Had various accidents with the car (though lucky enough to escape with not even a scratch!) and hence the car has spent a lot of time getting fixed (see General Comments...).
1 wheel bearing changed under warranty.
Bad choice of batteries in the early years caused problems, including the only time the car had to be towed.
Again in its early life, a very weak idle caused the car to stall very easily. Had the car checked and repaired. Still the problem has never gone completely away. The thing is that this car needs to be warmed up really fast after a cold start, because the cold higher idle (about 1500rpm compared with 1000rpm warm) drops REALLY fast, which means that if you live in a city center like me, there's no time for the engine to warm up, and as a consequence, it's difficult to keep it alive at traffic lights. In other words, after you've started the engine in the morning, you need to sustain your revs above 3000 for ten minutes at least. This thing has never been really acknowledged by the service people, and I would like to hear from other GTti owners their views.
Some minor worries with the interior:
Base of the passenger window lock switch has gone off. Fixed it myself with silicone.
Various creaks from the dash when cold seem to have finally gone over the years.
Passenger electric window is painfully slow, especially when you try to wind up both windows at once. It seems to have gotten even slower recently.
Other than that, standard maintenance.
I really love this car. It's really very uplifting to drive it and hear the engine scream to 7500rpm (sound proofing is nearly non-existent also). At 2500rpm you start to feel the acceleration, but at 3500rpm you get a real kick at your back close to what you feel inside an airplane at the moment it takes off! Imagine 0-60 mph in 7.9 seconds from a 993cc engine! (Autocar magazine test). That same mag listed this car at its all-time-greats page! However be warned: this car is a 1987 design, and there is no sophisticated electronic gadgetry to regulate the power and torque. This makes it difficult to control the power delivery, and sometimes even dangerous, especially for a young driver like I was when I bought it. You have to be VERY careful with it when you drive fast in twisty and/or slippery roads. The slow unassisted steering for such a fast car ads even more difficulty.
Back in 1994/95, I was also considering the Peugeot 106 Rallye 1.3, which was all the rage back then, and had the same power to weight ratio (but is slower to 100kph due to worse low-end torque). In the end, I decided to choose instant thrills against superior handling, which in the end could lead to taking more risks...