I'd like to have checkpoints, but I understand why they aren't included. You could create ones that are impossible to leave with small Mario (build Brick Blocks around it for example).
Star Coins would be nice, too, but the moderation process may be too complicated.
And it's kinda annoying that the levels of the people of your friend list aren't accessible directly. It's ok for me because I have miiverse on PC, but people that only use a Wii U have to write down the code...