Dont get me wrong, Brawl is a fun game and all, but it doesnt compare to its ole grampa on the 64. I never really got into melee so I wont discuss that much.
Reason one - Reflector Sheilds. I mained fox in the 64 version. why? well he was one of the better players, good speed, good throw, my friends hated the laser, etc. But fox had something noone else had, the reflector sheild(ness could reflect stuff with his bat but no1 could consistently do that). Now in brawl anyone thats anybody can reflect crap. its almost like your unique if you dont have one. some fools even have multiple ones. On brawl I have lost all interest in playing with reflective characters.
Reason two - Stages. They were so much better back then. Simple, yet all unique. not many maps can match hyrule castle, fox's spaceship, or the kirby level with the tree. not to mention all the other great ones. They let you play melee maps in brawl which is nice, but not the original classics!
Reason Three - Stuntimes. I never really got into all the instacombos in the 64 version but I strung together some small combos here and there. i mean in brawl sometimes if you hit someone and your move has long recovery people can hit you back before you can run or sheild or hit again. wtf.
Final thoughts - Learning curve. the 64 version required decent skill to actually be good. Those with lots of practice will totally destroy those that are somewhat new to the game, even those that are decent. Anyone can be halfway competitive at brawl. My bro had the game for a couple months, I came home and could almost beat him after a few times playing (he still beats me almost every time but thats because he only plays no items, calm levels, to take out all chance. at least I get close most of the time)
You should get to play 64 on wii. Man I would never play brawl again.