
Facts about the game:
Brawl is the third game in the Smash Bros. series and will launch in Japan in January and North America on March 9th. The game's website, smashbros.com, is updated every weekday with new information.
Controls:
You can play using either the Gamecube controller, Wii Remote by itself, Wii Remote with Nunchuck, or the Classic Controller. Controls are customizable. Everyone should be satisfied here.
Third party characters:
Sonic the Hedgehog and Solid Snake are confirmed to be in the game. As of November 7th's smashbros.com update, third party assist trophies have been confirmed, starting with Metal Gear's Gray Fox.
Subspace Emissary:
Brawl's version of Adventure Mode. You will battle across various Nintendo worlds on a mission to stop invading monsters/rescue princesses/retrieve Donkey Kong's bananas. This mode can be played cooperatively, but as far as we've seen you can only select certain characters for each mission as the story dictates. ROB, the old NES helper robot, is confirmed to be an enemy.
Online play:

Yes, there is online play in this game. Up to four people can fight it out online, and you can send each other four pre-written messages. Since this is Nintendo we're talking about, they'll find some way to hurt the experience. We already know that there's no way to view other players names online unless they're on your friends list, which is ridiculous since you could in every single other online Nintendo game. There is also no record keeping, which strikes me as incredibly bad, as keeping an online win/loss record is great for matchmaking people of equivalent skill. (Source: Smashbros.com: "Your opponent will not know your name or any information about you, and neither of you can send short messages. There will also be no battle records kept for this mode, so whether you win or lose, it doesn’t matter. Just sit back and play.")
Assist Trophies:
These work the same as Pokeballs, but instead of Pokemon, characters from a huge variety of Nintendo games appear. There is not a full list yet, but we know that there are assists from popular games such as Advance Wars and Fire Emblem, nostalgic classics such as Excitebike, Punch-Out and the SNES Sim City, and obscure titles such as Devil World.

Final Smash:
A significant new addition to Smash Bros. is the Final Smash attack. During battles, a Smash Orb will occasionally float by. Break it open to gain access to your character's Final Smash, which is performed by using your Special Attack button after getting the orb. This attack varies by character but for the most part is either massively damaging or has significant status effects. Bowser's, for instance, results in this:

You can only use one Final Smash per orb, and if you take too long to use it it can be stolen from you by other players.
Stages:
There are a variety of stages in this game, but what's most notable is that there is now a stage maker mode! You can design your own maps, save them, send them to friends, or submit the to Nintendo. Periodically, Nintendo will post the best stages for everyone to download, but they won't be hosted forever.
Characters:
A huge number of new characters have been confirmed already, and there are more to come. Not all of Melee's characters have been confirmed, but given their popularity, it's safe to assume that most of them will return.
New Characters:

Pit - Kid Icarus

Wario - Super Mario Land 2, Wario Land, Wario Ware

Zero Suit Samus - Metroid: Zero Mission

Ike - Fire Emblem

Pokemon Trainer - Pokemon series. Note, the trainer does not actually fight. You play as Charizard, Squirtle, or Ivysaur and can switch between the three at any time.

Diddy Kong - Donkey Kong Country

Metaknight - Kirby series

Solid Snake - Metal Gear

Lucas - Mother 3

Sonic the Hedgehog - Sonic the Hedgehog series, nightmarish fanfiction

King Dedede - Kirby series

Captain Olimar - Pikmin series
Returning Characters:

Mario - Super Mario Bros.

Link - Legend of Zelda

Kirby - Kirby series

Pikachu - Pokemon

Fox - Star Fox, even more nightmarish fanfiction than Sonic

Samus - Metroid


Zelda/Shiek - Legend of Zelda

Bowser - Super Mario Bros.

Donkey Kong - Donkey Kong series

Yoshi - Super Mario World, Yoshi's Island

Princess Peach - Super Mario Bros.

Ice Climbers - Ice Climber
Note: This topic was split from the Golden Sun thread, hence the first few responses. The first response was in reference to me saying that I can't believe people are angry about the lack of playable GS characters in Smash.