There are a couple of things to consider:
1) crowds are really low around Labor Day
2) that's because it is at the height of the hurricane season - though the odds for hitting on a specific day is small. It has happened though.
3) It will be hot
4) Food and Wine starts on September 14th this year so expect something like that for next year too.
5) F&W will bring in locals and foodies. Much of it is centered in Epcot, but there will be bleed off to other parks
6) The "
free dining" promo blocked out all of October and the first half of November.
If you can handle the heat, the first couple of full weeks in September will likely have the lowest crowds.
I will second PrinceOfPeace's recommendation. The two sites will give you a lot of information. If you don't want to have to search through the websites, you can buy there book which is specifically designed to walk you through the planning process. I have been to Disney many times, but still bought the book. It's a slightly different view than some other guide books and presented with a bit of sarcasm which I enjoy.