1) HEAT -  Yes you can actually have cabin heat in the winter if you have a water cooled auto engine vs. an air cooled airplane engine.

2) PRE-HEATING SUCKS -  No matter what series of electronic or fire breathing contraptions you have, pre-heating an engine in the winter is a huge pain PLUS if you actually go somewhere you can't stay long enough to let the engine get cold because you can't realistically pack your Tanis pre-heater and 20 gallon LP tank so you can pre-heat again the next day when you fly back.  Of course you can always drain the oil, take it inside with you and then pour it back in the engine the next day... yeah sure.

3) EXPANDED FLYING SEASON - When the temperature drops below 10 degrees it becomes simply cruel to fly an air cooled engine.  When you come in for landing there's no way to avoid some level of shock cooling on those front cylinders.  Not only do you get grounded for crappy cloud covered winter weather, when it's perfectly clear it's usually cold too so you can't fly on a clear day and you can't fly on a cloudy day... you can't fly three months out of the year.

4) RELIABILITY / LOWER OPERATING EXPENSE - people cite fuel burn, fuel cost, overhaul cost and things like that, but that's not what I'm talking about here.  You can buy an airplane engine that has been modified (derated) to use auto-fuel so that evens the fuel cost playing field and 2000 hours is a lot of flying before you actually hit overhaul.  BUT here's the trick, if you browse through Barnstorms and notice how many airplanes have had a "recent top overhaul" or some other major recent repair to the engine well before the TBO.  You'll be amazed.  Not only are parts more expensive, it's FAR more likely that you'll need them when you fly behind a "good old reliable airplane engine."  BTW - I've got three cylinders off mine right now getting new valves... nowhere near TBO.  Good old reliable... yeah right.

5) MODERN - While you can get airplane engines with EFI, a modern auto engine is going to be far superior to an aircraft engine in terms of design.  Remember, there was a time when almost all engines used magnetos... but now you've got to go to the antique gas engine show at the local fair grounds to see them.  I don't know how else to say it other than the auto industry has the engine part of the car figured out and the airplane guys don't.

6) MAINTENANCE - I believe that if I build the engine from scratch I'll be much more capable of performing maintenance, including maintenance in the field should the need arise.  If my memory is correct, most engine failures in auto conversions according to EAA studies are due to fuel flow, cooling and then ignition so using standard auto parts in those areas increases the maintainability of my engine if something goes wrong while I'm hunting in some little town in South Dakota.  They'll have an auto parts store, but they won't have an FBO with a mechanic.

7) SIMPLICITY/PILOT WORKLOAD - Most accidents are due to "pilot error."  Studies show that on average people can handle seven things at once before they start making significant mistakes.  Let's say you're flying in pure IFR, unknown territory, new airport, high winds, other traffic, ice starting to build up, you gotta pee, your kid is crying, your wife is praying that you don't die, etc, etc... do you really want to screw with carb heat and leaning out the engine correctly too?  I want an engine that's smart enough to know when it needs more air or more gas.  I want an engine that doesn't have the potential to have ice build up in the carb.  

7) HISTORY - I've had nothing but bad luck with airplane engines.  Every single annual I cringe and then there's this horrible sucking sound that comes from my wallet.  So that's my history but when I talk to long time pilots there's a high number who have experienced engine failures.  Now I'm not talking about guys who racked up hours as a CFI, I'm talking about guys who have been flying behind a single engine piston plane for recreation for a long time.  A surprising number of them have stories that involve engine failure.  Valves getting chewed up and things like that. 

Happy Flying