No other nation made as big a commitment. China did announce major goals of using clean energy such solar, wind and nuclear power, but they did not commit to a specific target for reduction.
Hatoyama's announcement of 25% is pretty ambitious and I am not sure they will be able to meet it. But I think Japan has a good chance of meeting that goal. But not just because of a strong push by Japan and Japanese to be more efficient and produce more clean energy. But because of Japan's rapidly shrinking population and relatively stagnant economy.
A country with a shrinking population will generally produce less waste and less emissions. So his announcement of reducing emissions by 25% by 2020, although impressive, is not as amazing as it would seem.