I think it's important to distinguish between different forms of colonization.

The New World was, for the most part, settler colonization with conquest and replacement of local populations occurring in the more populated parts of the continent. Africa and the Middle East were more political colonization, with the creation of new (artificial) territorial units run by Europeans who had no need or desire to settle there, and mainly occurred between the late nineteenth century and World War 2.

South and Southeast Asia, and parts of East Asia, were colonized after the New World and before the Middle East, and there we see two distinct phases - early mercantilism (c. 1500-1800), in which foreign companies, governments, and trading diasporas are basically just there to do business (and use force of arms to protect their interests) and full-scale imperialism (c. 1800-1945), in which European powers were either controlling or replacing local regimes, in much the same manner as Africa and the Middle East.

Most of the region was overrun, but a few locations - China, Japan, Thailand, and Nepal all being examples - remained independent, either due to their unity, as /u/ParallelPain has observed, or simply due to the fact that the Europeans figured it wouldn't be worth the trouble of colonizing them.

Of course, let's recall that ALL of these areas were affected in one way or another by early mercantile imperialism.

Please note: Because there has been a large amount of speculation masked as expertise on this thread, this is a reminder to please check the rules before posting if you're not already familiar.

Additionally, in this one case, I'm requesting responders to cite a source for all new top-level replies. It will help keep the speculation and guesswork to a minimum. Please don't answer with anything you can't back up with sources.

I reject the premise that China wasn't colonized. For one, we had the French concession in Shanghai, a valuable port city. It was definitely seen as a form of colonialism or imperialism. See Cady, J. F. (1942), The Beginnings of French Imperialism in the Pacific Orient, Journal of Modern History 14 (1): 71–8

What is a concession? Ceded terrorities governed by a foreign country. That's standard colonialism. That's how the UK acquired Hong Kong See *Têng Ssu-yü. Chang Hsi and the Treaty of Nanking, 1842. Chicago: University of Chicago Press, 1944. *

The United Kingdom, Russia, Belgium, France, Germany, and a whole host of other nations had concessions in China. Much of the interior was left intact, but only because the most valuable portions were on the coast.