A particular model year of car wouldn't be available until after that year - instead of before it.
Every time they repainted the lines on the road, you'd have to buy a new car.
Occasionally your car would just die for no reason, and you'd have to restart it. For some strange reason, you'd just accept this.
Occasionally, executing a maneuver would cause your car to stop and fail to restart and you'd have to re-install the engine. For some strange reason, you'd just accept this, too.
But that wouldn't work, you'd have to take the engine out, do nothing to it, then put it back in.
You could only have one person in the car at a time, unless you bought a "Car 95" or a "Car NT". But then you'd have to buy more seats.
Sun Motor Systems would make a car that was powered by the sun, twice as reliable, and five times as fast - but it would only run on five percent of the roads.
The oil, engine, fuel and alternator warning lights would be replaced with a single "General Car Fault" warning light.
People would get excited about the "new" features in Microsoft cars, forgetting completely that they had been available in other cars for many years.
We'd all have to switch to Microsoft fuel.
The U.S. government would be getting subsidies from an auto maker instead of giving them.
New seats would force everyone to have the same size butt.
The air bag system would say "Are you sure?" before going off.
The steering wheel would be replaced with a mouse and you'd need to memorize the keyboard short-cut for "brake".
For some reason the engine controller would need a 1G hard disk and would take 5 minutes to boot up.
They wouldn't build their own engines but form a cartel with their engine supplier. The latest engine would have 16 cylinders, multi-point fuel injection and 4 turbos, but it would be a side-valve design so you could use Model-T Ford parts on it. There would be an "Engine Pro" with bigger turbos, but it would be slower on most existing roads.
Your car would refuse to start with a message "Abort, Retry, Fail or Cancel?"
You would have to have a full service every 500 miles.
The speedometer would read 70 even though you are only doing 50.
They would make a flashy convertible model, where if you raised the top the engine would overheat.
The entire engine wouldn't be in the bay at once, and the car would have to keep stopping and starting to load in the relevant parts.
Every time you carried a new passenger you would have to alter the cars configuration settings. When the passenger alights these configurations would remain in place.