Smash Bros is supposed to be a celebration of Nintendo's history in gaming. That's its point, what defines it. Opening the door to all sorts of popular characters and references would devaluate it. It would lose all focus, becoming an empty shell of what it's supposed to be. Sure, the gameplay would still be there, but with the concept gone, the experience is not the same. It would be as random as those M.U.G.E.N. fan games you see around.
I love Goku, but I don't want to see him in Smash. I love Seinfeld too, but I don't want to see him either... I want to see characters that were born out of gaming, with a meaningful history on Nintendo platforms, both in time and popularity (around million-seller level, at least). A "Hall of Fame", if you will, rather than just a random popularity contest. And on a more personal note, I would prefer if the characters were limited to Japanese ones, and of fantasy nature. I believe these are the ones that fit the most on Smash due to their aesthetics, abilities and personalities, not to mention the fact that I have a preference for them...
Oh, and I do agree that Snake wasn't a very good choice for Smash... There's many other characters that were more deserving of that place. Obviously Mega Man did, and I was really happy to see it being recognized. Now, Bomberman is my most wanted 3rd party character, but I doubt it'll make it... Poor, neglected Bomberman...
PS: I would also argue in favor of Kos-Mos (Xenosaga) as a special case in relation with Monolith Soft and Namco, but I realize it's a hard bargain here...
