We were also there on Columbus Day weekend 2011 during a tropical storm. There were more people in October 2011 than there were the first week of Sept 2015 and both weeks had hurricane threats. We all know crowds in 2015 are much higher than 2011 so the only explanation is that Sept has incredibly low crowds. Yes, columbus day being higher than labor day shouldn't be a surprise, but according to some of the panic I'm reading here maybe it is.
Look at the crowd calendars at easywdw, yourfitstvisit, dads guide to wdw, etc. They all agree, September has the lowest crowds of the year. The other 48 weeks really aren't close (although between jan 5 and MLk can be low, too.)
I can't help but laugh at people above saying it's been as bad as July crowds. I can only assume they've never been in July!
Here's a quick test: if Haunted Mansion is not using the extended queue where you have to walk down the steps and along the dock before coming back up to the normal queue then you are in a low or medium crowd level.