First of all, I would like to say if you've never played Super Smash Bros. Melee, go buy it and do so now, I'll wait.
...
...
...
Done? Good. That was a really great game. But Brawl has surpassed it in every way and then some. Everything from the graphics to the unlockable content to the addition of online play makes this not just the greatest Wii game, but perhaps the greatest game of all time.
It's an insane fighting game in which you play as characters from virtually every Nintendo Series imagined, and anything could happen with all the items and interactive stages.

Gameplay
Man, where do I start? I guess I could start by saying that there's easily over 100 hours of content in here. There's 35 characters (including Snake and Sonic), 41 stages with a create your own stage option, and over 1000 unlockables, including trophies, stickers, and the new Chronicle. The Chronicle is a timeline of nearly all Nintendo games and when they came out. It's totally useless, but it's still executed wonderfully. The Masterpieces are demos of old games. Also totally useless, and the demos are way too short (you get 30 seconds for F-Zero), providing the only low point of the game. I should go into detail from here on, though.

Subspace Emissary
While the Adventure mode in Melee was just a beefed-up Classic Mode, the Subspace Emissary is a full-fledged, Castlevania-style adventure that you unlock all the characters in. You go through levels in a side-scrolling adventure in which most of the Nintendo characters are trying to fight a very dangerous evil. At the end of the level, you usually must fight bosses from previous Nintendo games (Meta Ridley, Petey Pirhana, etc.). There's also a way to upgrade your characters in that mode using the stickers you can get throughout the game. There's even a Co-op mode.

One Player Modes
If you did what I said to do and played Melee, then this will come as no suprise. Classic and All-star modes have returned. In Classic Mode, you just fight enemies across 12 stages with the rules changing a little bit every time. In All-star mode, you must defeat every character in Smash Bros. with only one life (on the hardest difficulty? Good luck). There's also Stadium mode, in which you do 3 activitiese for the highest score. That one wasn't my favorite. The one-player modes would be weak compared to Multi-player if it wasn't for the Subspace Emissary.

Multiplayer
As any Smash veteran would know, this is the ultimate party game (yes, even better than Mario Party, the original party game). You can adjust the rules of the fight any way you want. And the game's 1-4 players per fight. In rotation mode, you can have up to 16 players taking turns in groups of 4 fight, and Tournament mode supports 32 players in a classic Smash tournament (down from 64 in Melee. Oh, well. Were you really going to have 64 people in the room, anyway?), ensuring the greatest party gaming experience to date. There is also Online play, but I can't give it a score because I don't have Wi-Fi. *cry*

Items
The items are the best part of the game. There's over 100 items that can land on the stage at any time. They range from a Super Mushroom (make yourself a giant) to a Poke Ball (A random pokemon comes out and helps you for a little while). This is what made the past games and this one truly insane.

Stage Builder
Possibly my favorite new addition to Smash Bros. is the Stage Builder. You can use many of the parts seen in the game, choose from 3 backgrounds, and can choose which song to put on the stage. I love this, but more creation options would've made it even better.

Unlockables
This is what really launches the game sky-high. There's over 500 trophies and 700 stickers to collect. In addition, there's the all new Challenges mode, in which you get hints as to how to unlock the thing in the window (ex. Brawl 3 times on Luigi's Mansion). However, to unlock the hint, you must unlock the thing in the window next to it. Like I said, over 100 hours easily.

See? I could've just put all that in the Gameplay paragraph. Aren't you glad I didn't?

Graphics
Whoa! Is that an Xbox 360 I see before me? These graphics are so detailed and textured, that you might wonder. Unfortunately, the Melee stages haven't been updated, leaving some graphic dust on those stages, and, sadly, avoiding the 10. Rats...

Sound
Let me put this simply. There are over 200 songs in the game, including Star Wolf, the Mario Bros. 3 theme, and the Metroid ending. They are either redone great or the amazing original soundtrack. You get songs by collecting CDs as they appear on the stage. And the voiceovers are great as well.

Control
Even the often overlooked controls are done masterfully. The controls are simple and easy to understand. There are even 4 controller options to choose from the Wii Remote, the Wii Remote and Nunchuk, the Classic Controller, or, the Melee veteran's favorite, the Gamecube Controller.

This game is so jampacked with content, I had to dedicate 7 paragraphs to the gameplay. This is easily the best game released on Wii so far, and it will keep you occupied for a very, very long time:)