Here's the list on my 6,200 mile 2004 A4 GTO, purchased new in Sept. 04:
1. Drove off the lot and noticed only defog working; no dash vents. Found problem myself; split vacuum line under fuel rail cover. Trimmed out split part and reconnected. Problem fixed.
2. Passenger side front freeze plug leaking. Dealer fixed on second try. Still good to date.
3. Harmonic vibration at idle in "D" in warm/hot weather. Not cam shake but a constant harmonic vibration throughout whole car; gets worse with warmer ambient temps. Under 40 degrees ambient virtually no vibration at all; 100 degree ambient and the dash literally hums from the vibration; car feels like a big old magic fingers bed. After so many trips to dealer I lost count, no resolution. They have washed their hands of it; Pontiac says "normal operation." It's winter now, so there is no problem. As soon as temps warm, though, I expect this to return. I suspect there is a problem with the fuel management or something where it's not leaning out with warmer, less dense air at higher ambient temps. I may try a custom tune to see if it helps.
4. Both front strut rod bushings blown/collapsed. These fluid-filled bushings are amazingly fragile. One hard stop and they are blown, after which alignment continually changes with road conditions. Installed Noltec strut rod bushings (at my own expense) and this problem is now fixed. Had four-wheel alignment done (at my own expense) and fixed the strut rub problem on passenger side front tire.
I'm not counting the numerous designed-in problems (no rear wing delete option, cheap, rubbery HVAC controls, no dead pedal, barely adequate brakes, ridiculous rear seat access mechanism, easily scratched silver painted interior trim).
I have a real love/hate relationship with this car. All the problems (especially the unresolved one) really temper my attitude. If the car wasn't such a blast to drive, it would be gone by now.