In the past is hasn't been too bad. But with all the changes to the races, who even knows anymore. Getting to the parks on the morning of the marathon is always tough. Getting to MK or Epcot on the morning of the half marathon is tough. Now with the 5K race moved to Friday, I couldn't even venture a guess.
I know we won't be there in 2007.
The weather is another who knows factor. One year it was very cold, you needed sweatshirts/sweatpants. The year after it was 80 degrees or warmer, way too hot and too humid for a good race. Last year it was cold to start (not 30s cold, though) and warmed up to the 50s, 60s.
You need to pack for any weather situation - layer your clothing and if you are racing, bring along some disposable clothing that you don't need to get back. Just remove it and toss it as you run. My husband forgot his last year and I had to go buy him some cheap sweats from Wal-Mart the night before.