I read Wikipedia and this article seems to give most weight to this theory:
http://en.wikipedia.org/wiki/History_of_humanity#Modern_history
During this period, European powers came to dominate most of the world. One theory of why that happened holds that Europe's geography played an important role in its success. The Middle East, India and China are all ringed by mountains and oceans but, once past these outer barriers, are nearly flat. By contrast, the Pyrenees, Alps, Apennines, Carpathians and other mountain ranges run through Europe, and the continent is also divided by several seas. This gave Europe some degree of protection from the peril of Central Asian invaders. Before the era of firearms, these nomads were militarily superior to the agricultural states on the periphery of the Eurasian continent and, if they broke out into the plains of northern India or the valleys of China, were all but unstoppable. These invasions were often devastating. The Golden Age of Islam[114] was ended by the Mongol sack of Baghdad in 1258. India and China were subject to periodic invasions, and Russia spent a couple of centuries under the Mongol-Tatar yoke. Central and western Europe, logistically more distant from the Central Asian heartland, proved less vulnerable to these threats. Geography contributed to important geopolitical differences. For most of their histories, China, India and the Middle East were each unified under a single dominant power that expanded until it reached the surrounding mountains and deserts. In 1600 the Ottoman Empire[115] controlled almost all the Middle East, the Ming Dynasty ruled China,[116][117] and the Mughal Empire held sway over India. By contrast, Europe was almost always divided into a number of warring states. Pan-European empires, with the notable exception of the Roman Empire, tended to collapse soon after they arose. Another doubtless important geographic factor in the rise of Europe was the Mediterranean Sea, which, for millennia, had functioned as a maritime superhighway fostering the exchange of goods, people, ideas and inventions.
Lieberman, in Strange Parallels deals a great injury to this outmoded theory. Lieberman does so by pointing out how few major states there were in Europe by 1800, and how that number had been decreasing (read: increasing consolidation) for centuries. Nonetheless, there are bigger holes in the theory:
Let's start with China: China is covered in mountain ranges - sure there are incredibly productive river valleys, the Yellow, Huai, and Yangze, but these cut through mountains almost from start to finish. The great Sichuan branch of the Himalya, the ripples of karst covering much of the south, and the various moderate ranges surrounding the yellow river almost to its delta make china a forbidding, formidable geographic terrain. It has also made China, for about half of its history since the Zhou, a very difficult place to control as an empire, and so ruled by a series of small states, just like the above description of Europe.
But Empire did persist in the last 1000 years, with the Ming, Qing, Republic and PRC - so the mountains weren't a deterrent to empire as the above theory suggests.
Second problem - it completely ignores the problems of resources in Europe. 1. The Mongols turned back from european invasion for two reasons - the expeditionary force sent there found little quality grasslands for their horses, and 2. there wasn't much there to take. The rich ag/population bases of the fertile crescent, by contrast, were big takes for an expanding empire.
So, why were europeans expanding? The same reason they weren't attractive to the mongols, they were resource-poor. The greatest mariners of european expansion were genoese - a state built entirely on middle-man trading. The burgeoning states of western europe grabbed up as many genoese as they could to expand, the portuguese for african gold, the English and Dutch for a way to the rich east, etc etc.