It's very much regional. In most places, government/publicly funded stuff goes down - but in many, regular business goes on as usual (even at financial institutions, etc.) - in fact, I was reading the other day that only about 1/5 of private business observe it. The new trend at a lot of private companies is to skip it and instead take 2-days at Thanksgiving, turning that into a 5-day weekend.
I think at WDW it manifests itself like Columbus Day, etc. - more regional/"local" visitors coming for a few days, versus planning a week's vacation - so that day itself may be busy, and the day or two on each immediate side, making it more of a blip than a chunk of time.