I remember seeing some pictures from TPR when they were at a theme park in Japan and the night before they got several inches of snow, but the next day the park was open and employes were shoveling and rides were running.
That is how is used to be here but now we close things way too early if there is even a threat of snow or inclement weather.
Well, my theory is that you can always put on more clothes if it gets cold, but you can't take any more off once you're down to a swimsuit at a waterpark...
I always love those days at the water parks that people consider too cold. On our first trip to Typhoon Lagoon, nobody would enter the slides or pool, only the snorkeling reef since it was warmer. Our family literally had the entire park (except the reef) to ourselves.
As far as destination parks go for me personally if it is Disney 50's aren't too bad but for Universal it has to be in the 60s because of the number of outdoor rides.
We also hit WDW 2 January's ago when it was down in the 30s. I had forgotten to leave my wool coat in teh car when I left Oregon, and boy was I glad! I felt sorry for many cast members that were wearing light costumes, they looked pretty misrable. No rides were closed, and as aways had a great time. It some ways it was a nice contrast to the August weather from our typical visit for free dining.
Rain Shine Day of an earthqauke (Disneyland) if the park is open I'm going
Sunday was in the 30s most of the day. I hated the weather, and had to wear two sweatshirts plus mittens, but my entire family still had a great time. The other two days were only barely warmer. Of course it was finally warming up yesterday afternoon and that's when we had to leave!! :-(
The best plus of the cold weather -- there were only 2 long lines the entire three days we were there, and one was because I screwed up our fastpass strategy. We got a TON done in a very short time, hitting almost all the attractions in Magic Kingdom, Disney Hollywood Studios, and Epcot in 2.5 park days.
I will always prefer 70 degrees or more, and for an "average" theme park might vote 50 or lower as too cold, but for WDW I'd have to say it's never too cold!!