September is actually our favorite time to go. The parks are less crowded, it's much quieter. I kinda like that type of atmosphere. You can actually have a DW vacation and relax a bit (that's hard to say for the rest of the year).
I do know that since it is an off peak time that parks close earlier and there may be more attractions closed for refurbishing. However, there are so many things to do at DW this shouldn't even affect your trip.
It is still hot, but it's hot from May to September anyway and with less crowds it's a little more bearable. However, I do recall a few years ago when we went right after Labor Day it was beautiful (70s and not humid), which is unusual but sometimes possible. I'll never forget that year because we were so used to it being so hot then and it was so nice.
The only downside as has already been mentioned is it is the peak of the tropical season which means this month pretty much has the highest threat of hurricanes, tropical storms and depressions. It seems like everytime we've gone in Sept. there was always some type of storm pondering in the Atlantic deciding if it was going to impact FL or not, but we always got lucky.
We're heading to DW this weekend with FL Play 4 passes, since we won't be able to use all 4 days I'm going to try to talk my dh into going again for a weekend this Sept too!
<font color=red size=5>Tomorrow, tomorrow....at this very moment we'll almost be there!</font></font>