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They really lucked out on in a geographical sense by having two really long rivers that ran parallel latitudily.  (?)  North/South rivers aren't quite as good because you get a much more varied climate going in those directions and its pretty much impossible to domesticate certain plants and animals across the country if it is a vertical type.  Having them be parallel made it easier to connect through canals.  Those Chinese were definitely not scared of giant public works projects. 

Speaking of public works projects, there was a funny line in this weeks Simpsons episode where the grandpa was having a flashback to his time in WWII and he's on this deserted island and he finds Santa Claus and says, "What in the name of the Tennesee Valley Authority!?!" 

I think the 125-year argument works for Japan because they had a radical shift in national identity after the arrival of Commodore Perry in the 1850's (Tokugawa to Meiji restoration).  China's major identity shift is mostly a result of the adoption of Communism after WWII.

Edited: one decade off on Perry...also, many of China's technological innovations are readily apparent, such as the advent of paper, gunpowder, and a good calendar.  Not to mention the Wall. 

Oh, I didn't realize you meant dynastic shifts.  I'm not really too good with Chinese history either...much better with Japan.  Japan is a lot easier because they were always way more isolationist and have a relatively homogenus history.  Same imperial lineage going back to before 1000 A.D.  Of course, there were changes over the course of events, but to keep the same imperial lineage going that long is something of an anomaly in the history books.

I could never keep all the Chinese dynasties straight.  Too many and they all sound the same, you know?