I dropped by Expo 2010 in Shanghai. The big country pavilions all had lines of at least 2-4 hours! So I didn't bother going to any of those, and I only went to the stuff no one cared about. :)
My favorite building is one of the few that will actually stay after the rest are torn down: The UFO-shaped Shanghai World Expo Cultural Center (next year to be renamed the Mercedes-Benz Arena; so much for culture):
I thought the South Korean Pavilion was quite clever, with Hangul integrated into the geometry of the building:
I'm not sure if these kung fu kicking sculptures were even part of the expo, but I thought they were awesome:
And last, but not least, the Oil Pavilion!
Those walls are actually giant video screens. At one point, they displayed, in English, the Expo 2010 motto "BETTER CITY BETTER LIFE" followed by "OIL EXTENDING CITY DREAMS".
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See the full set here.