I'm a hot sleeper, but I give a lot of thought to why people are so different. I haven't actually done any research on this, just a lot of thinking. Here are the factors I think come into play:
1. Metabolism - it seems the slower your metabolism, the colder you'll be, though this makes no sense to me. In fact, I'd consider my metabolism to be fairly slow. I usually eat only one meal a day, but I'm a hot sleeper.
2. Body type or somatotype (endo, ecto or mesomorphic): I think I'm somewhere between an ectomorph (low fat storage) and a mesomorph (medium), but one would think the endormorphs (high fat storage) would be the warmest. However, It seems the chubbier types are always cold people. Very counterintuitive.
3. Genetic background - the Celtic, Scandinavian and Germanic (and Eskimo, I'm sure) peoples have a definite advantage on Mediterranean and equatorial cultures. We've had to adapt to cold weather for a much longer time since our people settled in colder climes. I'm Irish and German so cold weather is in my blood.
4. Mental attitude - I've noticed that cold people think about being cold a lot more than normal. You know the type - it's 35 degrees outside and they're dressed for subzero temperatures. That's T-shirt weather to me (unless I'll be out there for a while). Unless I'm gonna be out in the cold for more than 15 minutes, I don't even wear a coat or gloves down into the teens. A lot of it is psychological - if you think you're gonna get cold, you get cold, and if you don't you won't.
You see this positive attitude toward cold weather in people who work outdoors a lot. They don't let their brain run wild with "cold" thoughts.
5. Experience - the more you spend time in cold weather, the less likely you are to get cold.
6. Sleeping habits - the cold sleepers want and need to be toasty warm to get to sleep, whereas the hot sleepers like me probably prefer a nip in the air for sleeping.
7. REM sleep - the average sleeper experiences about 25% of REM sleep per night. I think hot sleepers have a much higher percentage of REM sleep. As soon as I kick into REM sleep (usually about three hours into the night) and start dreaming, I get hot as hell and start kicking off the quilts and trying to get some ventilation.
I don't mind being hot 'cause I love the dreams that come with REM sleep - it's better than watching TV!