What makes this a typhoon and not a hurricane? Or a tycoon?
Location. It is effectively, if in the Atlantic or eastern Pacific, a hurricane.
I have always understood them to be identical, but several definitions I've seen include qualifier words like "effectively", "virtually", "practically" and similar. Are there any actual differences—not mere tendencies, mind you—but qualities, say of a hurricane, that if it suddenly appeared in the western Pacific, it would not be termed a tyhpoon? And vice versa.
They're exactly the same thing. Upon moving past the 180 meridian, a storm will be redesignated. The generic term for all of them is "tropical cyclone." In the Indian Ocean and in the Southern Hemisphere, they're called cyclones. These also are the same thing.
Western Pacific storms tend to be the biggest, in part because the Pacific is bigger so storms have more time to grow. I suspect there are other factors such as sea temperatures and wind patterns conducive to large and powerful storms there. Hurricanes are classified on the Saffir-Simpson Scale, while typhoons are either typhoons or super typhoons. A super typhoon is one with winds of at least 150 mph, equal to a category 5 or high-end 4.
Tropical cyclones may behave differently in different places, based on the prevailing environment, but they're all exactly the same species. They're not "essentially" the same thing; they're exactly the same thing, living in different environments and given different names.