There is a difference in Copyright and Trademarks. The Copyright is literally the right to copy something, and those rights do expire, theoretically well after the original creator has passed and no longer has an interest in profiting from the work. So, in the case of a Copyright expiration, people can either copy and publish the original work and sell at their own discretion, or even create derivative work based on that, for example use Winnie the Pooh in a movie like that. This however does not extend to Trademarks, which are well protected. Trademarks are things like logos and other such branding. A person will not be able to use the name in titles and such. Like, they could likely release a DVD called "Steamboat Willie - the Original Mickey Mouse cartoon," but they couldn't just call it "Mickey Mouse." They could not use the Disney logos and trade dress. In the case of Winnie the Pooh, remember that Disney does not own the copyright for the character - the A.A. Milne estate does. It will become no different than a fairy tale like Cinderella - anyone can make Cinderella, but they CAN'T make the Disney version or something too similar, (blonde hair, blue/silver dress, etc.) becuase they still own it (until that falls into public domain, but that one still has a while). In the Winnie the Pooh projects, they might not give him the red shirt, etc. as that is part of the Disney styled version.
Anyway, that may be oversimplified, but that's basically it. While I actually support copyright extensions for still in-use IP like Mickey Mouse, it is unlikely that it will actually impact Disney's profit in any significant way and derivative projects are likely to be few and far-between. Most people who support copyright expiration just want to archive and preserve all works of art and not be at the whim of a big corporation to do so, which I can see that too. Really, it's not that big of a deal either way.