Okay, so.
-Subspace Emissary is the new Adventure Mode.
-It's awesome.
-There are tons of new locations, partially with old elements ( Bullet Bills ), partially with completely new stuff ( or has anyone ever seen that freaky GOAT before?! O_O ).
-The mystery things come in masses out of the Halberd, and are enemies. Metaknight, however, is a protagonist, as he's seen fighting monsters.
-THERE ARE A FRIKKIN LOT OF ENEMIES WITH AI AND EVERYTHING!
-Hammer Bros are also enemies you can fight. That raises the possibilities of Assist Trophies being playable, as they seem to possess movement abilities and certain animations you just need for the game to work.
-It's well possible that every character will have a certain own storyline, with their own intros. This is, however, not guaranteed.
-Mario and Kirby DO NOT break the rules, it seems.
-Peach is in.
...that's all.