Brawl is already deeper than Melee, and there's nothing Melee can do about it. It seems hard to believe that in about a week I can come to this decision. But remember, because I've been following Brawls design paths for so long, it's as if I've been playing Brawl for months.
I personally believe that Melee wasn't balanced at all. It's not that it was unbalanced, I believe the developers didn't concern themselves with balancing the characters at all. This time around, Sakurai has crafted each character with his own hands (literally [kind of]) putting a significant amount of time and focus on balance. If a character has a really good kill move, there are drawbacks. They might not have many kill moves, it might not have a lot of range, they might only have kill moves that kill horizontally or vertically which in turn shapes how they approach their enemies. If a character is really fast, they're generally weak forcing them to chase down their opponents repeatedly for kills. I've already noted a variety of ways really strong moves and characters have drawbacks designed right in. Beyond this level of balance, Stale-move negation keeps players using a variety of moves because the effectiveness (damage and knock back) of each move will weaken dramatically if used in succession. What's even more exciting about having to use a variety of moves to round out one's game is, every move is effective in Brawl. From the neutral a punches to the smallest low tilts, every move can be used. In Melee, crouch canceling and L-canceling put a lot of emphasis on the air game while greatly reducing the effectiveness of the ground game. Tilts couldn't typically be used against a crouch canceling ready to down smash opponent. Melee's design essentially took away one's ground moves and turn them into a shffling air attacking junkie. But with Brawl, there's no L-canceling or crouch canceling, plus the range of DI from attacks have been tailored for each attack. This means you can't DI all my attacks 45 degress up all the time. Now, when I forward tilt you, you go forward. Now when I low tilt you, you slide out and maybe even trip up. Furthermore, there were many moves in Melee that became increasingly ineffective when used on opponents of higher and higher damage percentages. After all, when the move doesn't do a lot of damage, it doesn't kill, and the opponent keeps DIing it up and away, then it only serves to stall the game out. In other words, in Brawl attacks just work. Many have cried about the loss of their beloved technique L-canceling. I'm not sure why they're so upset. The air A moves in Brawl auto-cancel, which basically means they cancel themselves. In reality, this is no cancel at all, but the speed at which some moves can finish with their animation is just like L-canceling. Back in Smash64, L-canceling canceled all the lag. In Melee, it cuts the lag in half. I believe this was an attempt for Melee to keep some of the moves relatively slow even when l-canceling, like Link's down air. Brawl does the same thing while reducing the need to slap a button upon landing. Most very strong moves take time to recover, while most quick weak moves take no time at all. Brawl is built this way to keep gameplay smooth, focused, and uncluttered without manic button presses. Also, there's no way to successfully have Melee's L-canceling with online play. The packets of information that would have to be sent as your character hits the ground and l-cancels could easily get lost in the web, not to mention the timing for doing so would be inconsistent depending on the connection. The new auto canceling air moves makes the air a wonderful place to be. However, shield grabbing is still in effect. What's new with Brawl though is that attacks can not only stun shields but make the opponent slide back consistently. When this happens the opponent is usually too far away to grab you. The best way to send your opponent sliding back is with smash attacks. In Melee, smash attacks could be shield grabbed fairly easily. But now, a greater emphasis is placed on smashing. Perfect Shielding is the new way to defend yourself and turn the tables on your opponents. After successfully power shielding in Melee, dropping the shield and retaliating took too long to be viable. Furthermore, because the speed of the game was so high because of L-canceling, the opponent could pour on the pressure without fear. Additionally, because the timing window for a power shield was just frames before an attack hits you (a great enough risk as it is), the critical moment to strike would be gone in a flash. In Brawl, the timing is easier not only because shielding is on a button instead of a gamecube trigger, but also because the timing has a bigger window, player can better prepare for the counter attack. To counter a perfect shield, multi hitting attacks or rapidly hitting attacks fit neatly into the small gap of time the opponents have to retaliate. This gives more importance to the use of the neutral A attacks. To mix up opponents who perfect shield a lot, the new momentum based air dodge has virtually no lag when the character hits the ground. This makes it possible to fly through or behind an opponent and find an opening instantly with an attack. However, to counter this approach, up tilts and up smashes now cover a little area in front and behind all characters, so falling opponents can't easily drop right through them and score free hits. Wrap it all together and you have an amazing new game that exceeds Melee by many times. But wait, there's more. Brawl has a 10 frame buffer window between attacks. What this means is, if you want to go from one move/attack directly into another move/attack as soon as possible, at the last 10 frames of first move/attack, if you input the next command, the character will start it as soon as the first move is over. This means, to play a character cleanly, quickly, and tightly you don't have to sit down and memorize a book of frame data. Now you can feel out your character's lag and focus more on having fun. Anything that takes the chore out of a game and keeps players playing is a better design choice. If anyone is upset that Brawl has this wonderful feature, then they can go play another game. In a world of well designed games and Smash, Brawl is king. And that's the biggest difference you need to know.
For the most part the game plays and feels as lot like Melee, that is to say it's still smash. Right off the bat, Brawl is a lot cleaner. The clutter in Melee stemmed from from a high game speed, invincible frames, and other invisible aspects of the gameplay (like L-cancels). All of those areas have been addressed. Upon first seeing Brawl in motion, I thought the game speed was dramatically reduced. Now, though the air fall speed has been decreased making all the characters appear to be "floaty," the game speed is actually just as fast or (could it be) faster. Sakurai commented long before that Brawl has a new air game. And true to his word, the aerial battles and ballets are nothing short that exhilarating. Because how much a player can DI away from attacks and the stun time from attacks has been reduced, more attacks and dodges can fit in a single aerial encounter than ever before. Also, footstool jumping allows both players to extend their air time/options by jumping off of each other. This new, non-aggressive way to interact in the air essentially extends part of the "ground" game into the skies. And with each option flows a triangle of balance that are tied seamlessly into the greater flow of the battle. And that's just removing the clutter.