Ooh boy. OK, here we go.
Characters- 51
Mario
Luigi
Peach
Bowser
Bowser Jr.
Yoshi
Donkey Kong
Diddy Kong
Wario
Link
Zelda
Shiek
Ganondorf
Ghirahim
Young Link (LTTP2 style)
Kirby
Metaknight
King DeDeDe
Samus
Zero Suit Samus
Pikachu
Jigglypuff
Lucario
Pokemon Trainer- Squirtle
Pokemon Trainer- Ivysaur
Pokemon Trainer- Charizard
Mewtwo- Standard Form
Mewtwo- Awakened Form
Genesect
Pit
Palutena
Hades
Marth
Ike
Chrom
Fox
Falco
Wolf
Ness
Lucas
Captain Falcon
Captain Olimar
Mr. Game and Watch
Ice Climbers
ROB
Takumaru
Shulk
Sonic
Snake
Professor Layton
Pac-Man
Zelda/Shiek, Samus, Pokemon Trainer and Mewtwo to be handled by transformations.
Assist Trophies
Groose
Fawful
Tempura Wizard
Dillon
Tempo
Emmy
Pac-Man Ghosts
Wonderful 101
Denpa Men
Miis
Pokeball Pokemon
Victini
Meloetta
Oshawott
Excadrill
Scraggy
Tornadus
Crustle
Sigilyph
Emolga
Zoroark
Stages
Space Pirate Ship- Kid Icarus Uprising
Spaceship Mario- Super Mario Galaxy 2
The Sealed Grounds- The Legend of Zelda: Skyward Sword
Wuhu Island- Wii Sports Resort
Phaaze- Metroid Prime 3: Corruption
N's Castle- Pokemon Black and White
Gaur Plains- Xenoblade Chronicles
Boxing Ring- Punch-Out!
Nintendo Land Plaza- Nintendo Land
Yarn World- Kirby's Epic Yarn
New Items
Bee Mushroom- Super Mario Galaxy
Magnus Club- Kid Icarus Uprising
Tanooki Leaf- Super Mario 3D Land
Gust Bellows- The Legend of Zelda: Skyward Sword
Takamaru's Shuriken Stars
On a side note, about E3 2013, brace for the worst: It's possible they may only show screens (which is what Iwata indicated they'd show, specifically saying "the first screens"), and that they may only show updated models for Mario, Link, Kirby and Pikachu. Of course, they'd do so knowing fully well how disappointed fans would be, but I wouldn't put it past them.
Sakurai tweeted that they were planning on showing a trailer, as well as showcasing the 3DS game. I wouldn't worry yourself too much.