As the most frequent forfour owner here...
Quirks first:
1) Self centering steering - Erm... no, on tight turns the steering won't right itself.
2) Bleeps - the forfour bleeps at you for almost anything!
3) HU - have a tendency to eat or spew CDs and cut out under excessive load with the softouch (ie kick down).
4) Softouch - Sports Start, from stationary foot flat to the floor in auto mode (please note, use a dry, straight and safe road to try this as the steering can pull and ASR/TC tries to hone the power).
Faults (for some reason around 44k miles):
1) Leaking shocks - easily dealt with.
2) TVA (Throttle Valve Actuator) - Either replace (costly) or clean (fiddly)
3) Softouch - crunchy nut gearbox easily solved by updating the software or cleaning the actuator.
4) Manual - Crunchy reverse, ensure car is fully arrested then select reverse.
Erm... more so check out FFO!