Woolly World succeeds Yoshi's Island, Yoshi's Island, and Yoshi's Island as Yoshi's stage in Smash Wii U, besides that terrible Melee stage. Woolly World is the Roy of Smash 4, as it's a stage from THE FUTURE! And not in the way that the F-Zero stages are in the future, I mean that Woolly World is based on Yoshi's Woolly World, a game that isn't even out yet. Sakurai probably did this because Yoshi's New Island was a huge disappointment. Can't blame him.
Woolly World is a dynamic stage, like most of Smash 4's stages. It starts out in some grassy (though the grass probably feels like pants) field with a windmill in the background and a bunch of clouds that the fighters can fight on. After a while, the stage will raise up, so fighters have to go on the clouds. The stage will then go to some area with blocks, I have no idea what it's supposed to be because YWW hasn't released yet. It's mostly a cosmetic change because both areas are flat. Still, it's a pretty neat stage. I really hope Yoshi's Woolly World doesn't suck.