Well, you remember Sydney 2004? It would be the same, but the party would be much bigger as people around the world loved to see Sydney's Y2K fireworks one more time. Sydney 2004 was great fun, anyway.
About Beijing? Well, I think the Xinjiang Independence War of 2006 would still occur. It was just bound to happen, even without the media attention it had thanks to Beijing 2000.
Well, let's see what Toronto 2008 will bring. Athens is still pretty pissed of by the IOC even as they finally got the 2012 games.