Looking back, Melee was more serious than SSB64, but still had its tongue firmly in its cheek. The music was more refined, but had quirky Nintendo charm. As was already mentioned, the graphics were colorful and vivid.
Its as if Sakurai really wanted people to take Brawl seriously, that it was more than just a party fighting game.
The best way to describe Brawl would be this...
Its what a Nintendo game would look like if it was developed by a hardcore gamer.
*Note I still love Brawl, but I stand behind my belief that I miss the wacky feel of the original.