At the current standards, apparently the only hard requirement is a pulse.
- Turn too early/late past an arbitrary point in space
- Turn the wrong direction
- Descend too early past an arbitrary point in space
- Use the wrong altimeter setting
- Failing to recognize and troubleshoot a failed instrument
- Failing to recognize signs of hypoxia
Not to mention dealing with emergency procedures while you don't do any of the above
Here is an example from a few years ago where a 777 missed a mountain by a few seconds after some non-standard instructions by ATC
I don't think the comparison between a regular driver's license and Instrument / IMC flight is reasonable. Perhaps more accurate to compare to a Commercial Driver's License with a HazMat load
Or driving a bus
Even in the aviation world, an IFR rating is largely considered the most difficult to get.
I would also add that in aviation, most accidents / incidents do _not_ result from a single error, but rather the stacking up of errors, which granted, are almost all variations of 'pilot error'
In a plane you don't have that single goto option. You have to know the correct response to every possible situation and it's different in each case.
Plus, many mechanical failures in an aircraft will kill you. Most things in a car won't. You need to learn how to look after a plane, and what to check for.