Let's just consider the following:
Melee's gameplay was more fun, and there were more of your favorite characters in it, other than Wolf and Sonic. The game was a lot faster, and it was actually very hard to unlock everything, and even I haven't gotten everything...I just need one more trophy. Anyway, the whole bonus points idea was a little...off, but the event matches were actually very fun and made sense. Brawl's were just kinda out of the blue... And who could forget the epicness in the guy saying either MELEEEEEEEE! or Battle Royale!
On the other hand, Brawl's gameplay is slow, and very easy to get used to. When it comes to recovering from a big attack, this game is more lenient than Melee; Mario's up special got him no where in Melee, and now you can be a foot away from the edge and still grab on. Brawl's simple unlockables made it quick to get the characters you wanted, and while I'm there I'll just say that this game has more characters, which is something people hope to see. This game has third party characters, which is a great idea, or just not that good for those intense Nintendo gamers. Like Dragoomba said, that's why the game says "Nintendo all-stars"; now it's just all-stars or "your favorites."
Consider those, and you have what I think... BOTH GAMES AND THE FIRST ONE MAKE UP THE MOST EPIC GAME SERIES EVER CREATED, NOW GO PLAY ALL THREE RIGHT NOW OR ELSE I WILL SEND MARIO TO EAT YOU.