From an engine builder's standpoint, your carb tune will play the biggest role in how long your engine lasts.
Back in the 60's-70's when carburetors were still the primo way of fuel delivery, you couldn't expect an engine to last more than 40-50k miles. Anytime the choke is on, or anytime the A/F mixture is rich, it will wreak havoc on the condition of the cylinder walls. Fuel injected engines, on the other hand, will generally last much, much longer.
Gotta keep an eye on your fuel pressure (too high of fuel pressure can cause issues, most carbs like 5-6 psi) and gotta keep an eye on the idle A/F ratio, as well as the A/F ratio at WOT. Remember, different pump gas formulations can cause problems, even something like changing an air cleaner can disturb the A/F ratio. Excessive amounts of fuel will wash the
oil off the cylinder walls.
Otherwise, the other guys listed some very good things to watch out for. Also, generally speaking, the more radical the engine, the shorter the life expectancy, due to higher cam lift, parts flexing due to high horsepower, etc, etc.