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Presumably, they're characters that are either in sensitive personal/place names, or they're part of commonly used and politically significant euphemisms or idioms.

There's also possibly a punitive element of this: in addition to whatever issues they have with some search terms, the Chinese authorities may have decided to be trigger-happy with the banning as a punishment to Google for ceasing its cooperation on search filtering.

The Three Gorges Dam on the Yangtze river causes a lot of environmental damage. They dont want Chinese citizens to read about it.

In this specific case, the character is the last name of the former President of China and chairman of the Communist Party, Jiang Zemin(江泽民, http://en.wikipedia.org/wiki/Jiang_Zemin)