I'm told that the most common one was a complete and total lack of heat in winter - in fact could not even keep the windshield defrosted. Hard starting was a common one, too apparently.
A guy I used to work with LOVED the Smart car but complained incessantly about the lack of heat and the lack of guts - zero passing power, useless on hills. His was a "first year" one and it needed a LOT of bugs sorted out, but he had thought that since the (essentially identical) one had been
sold in Europe as the "Swatch" (Yes same company as markets the watches!) it shoulda had all the bugs out by then. Wrong.
He now has a gas one - loves it so much he even got another for his wife!
Their third vehicle is a 2004 Chev Duramax one ton dually that they use to pull their humongous fifth wheel