- Environmental damage and resource consumption - ie carbon emissions and the economic threat of peak oil. Using a car seems to be an extravagent use of a precious resource.
- Congestion - even if cars were using a renewable clean energy - you would still have a problem of traffic congestion - this wastes everyone's time. We would be a more productive society without it.
If we invented flying cars, and a clean renewable energy to power them, this would solve both these problems. Unlike in movies like the Fifth Element, there would be no intersections with flying cars; at the points where there usually would be intersections, the cars could just fly around each other (similar to how highway clovers don't intersect).
There is still an issue with resources I guess, if given we have the energy to run a flying car for everyone, we would still need the materials. But this seems like a problem that would easily be solved by free market allocation.