Gobs of torque, less maintenance, engines last longer, better milage, less harmful exaust output.
With modern fuels, modern glow systems and better injection systems cold starts and excess noise is no longer a problem.
If I came across a viable engine for cheap, I'd convert a Ranger in a heartbeat. I'd love to find a decent turbo diesel to go in a Ranger. I've seen a few suggestions of using a Powerstroke V8 or a Cummins 6, but both are too heavy to be good retro fits.
I'd want to find a light, long stroke 4 cylinder that doesn't weigh too much more than the gas engine.