I've been reading some grumbling at the expense of the inauguration, and that this would not be the reaction if John McCain had been elected.
It's simple--Barack Obama is the first African-American president and a young energetic politician who inspires for a number of different reasons. The combination of those factors has made his inauguration much more significant than probably any other in history. There have been a lot of firsts, but none so significant since the inauguration of the first president, George Washington.
Combine that with the sourness attached to the departing president and a deep economic crisis and you have the recipe for an exciting, even cathartic celebration.