* Fuel starts to turn into a syrup from evaporation requiring you to drain the tank. (This is a big issue on 2-stroke engines, you need to do a lot more work)
* Oil should be replaced due to acidic oil eating away sediment from crank case.
* Depending on time of year coolant should be flushed (depends on what mix is already in).
* Battery will be dead.
* Brake fluid might need to be changed as well.
Half of these you can ignore but it will cause wear and isn't good thing.
Typically? You'll probably have to jumpstart it. If it is sitting outside you could end up with the parking brake, in particular, stuck. It's not good practice to just walk away from a car for this long (should disconnect battery, put it up on blocks, etc.) but nothing terrible is likely to happen--especially if it's protected from the elements.
The running down of batteries in an electric car to the point where they're damaged is definitely a corner case. It's not one that's out of the realm of possibility. (Someone goes to winter house for a few months, leaves car in garage, and the charger isn't hooked up properly or something.) It sounds like the current generation vehicles make it even more of a corner case although I have to wonder what the failure modes of the various protection systems are.
If you disconnect one of your car battery terminals, it will be perfectly fine for six months. Reconnect the terminal and drive away. It's not necessary to do anything else.
The main danger is things like squirrels moving into your engine block, if it's outside.
That is the comparison I would like to see.