Most of the new features added (specifically for Super Mario World, 'cuz I know that's the theme I'll be using the most). I also like the new multiplayer (even if it's not fully finished yet).
The themes for that game are alright. The desert theme looks more like a chocolate island theme (which resembles much more of a mountain aesthetic). The night theme BGs feel weak as it's only just the stars... why not include the rest of the background from the daytime as well?
From what I've seen, the blocks that create paths and custom screen scrolling settings are my favorites. Once I get the game, I'll be toying around with those to see what kind of level I can make with those things.
Of course, I'm saying this ahead of time, and the feature I enjoy the most might change overtime.
Probably the Super Mario 3D World style because cat Mario, meowing bullet bills, directional pipes, and in general sensing a lot of creative potential. Still haven't quite explored every orifice of this game yet so my opinion is subject to change.