The Official List of Confirmed Stuff in SSB4!

Here's a list of everything confirmed:

Characters:

Veterans:

Mario
Donkey Kong
Link
Samus
Kirby
Fox
Pikachu
Bowser
Pit
Olimar
Luigi
Peach
Toon Link
Sonic
Marth
Zelda
King Dedede
Lucario
Diddy Kong

Newcomers:

Villager
Mega Man
Wii Fit Trainer
Rosalina
Little Mac

Stages:

3DS Version:

Battlefield
Super Mario 3D Land stage
Rainbow Road
Gerudo Valley
Spirit Train
Arena Ferox
Kid Icarus: Uprising stage
Nintendogs stage
Find Mii/StreetPass Quest stage
Animal Crossing: New Leaf stage
Unidentified stage, possibly based on Donkey Kong Country Returns 3D
Boxing Ring

Wii U Version:

Battlefield
Mario Galaxy
Skyloft
Pyrosphere
Palutena's Temple
Garden of Hope
Animal Crossing: City Folk stage
Wii Fit stage
Windy Hill Zone
Dr. Wily's Castle
Wuhu Island
Boxing Ring

Items:

X Bomb
Hocotate Ship
Steel Diver
Beetle
Capsule
Smash Ball
Ray Gun
Assist Trophy (Skull Kid, Nintendog, Starfy, Ashley, Mother Brain, Waluigi)
Green Shell
Fire Flower
Poké Ball (Palkia)
Home-Run Bat
Bottled Fairy
Banana Peel

Trophies:

3DS Version:

Saria

Other Characters:

Toad
Shy Guy
Lubba
Luma
Birds
Alfonzo
Navi
Phantom
Loftwing
Viridi
Kapp'n
Lloid
Dr. Shrunk
Gracie
Saharah
Wendell
Tom Nook
Xerneas
Male Wii Fit Trainer
Nintendog (Beagle)
Dark Emperor
Yellow Devil
Rush

Inform me if I missed anything.

a lot of this is already covered in the op of the main thread (though it's slightly out of date) with the exception of the other characters part