OK, let's analyze the 64 Kirby and see what made him so great.
UP-TILT! This was probably one of the most broken moves in Smash 64. Kirby's uptilt was the fastest tilt in the game, had huge range (huge feet + invisible hitboxes), and comboed extremely well.
Example:
http://youtube.com/watch?v=yK7WWPUP67k
Huge feet + invisible hitboxes. Kirby's feet were around 1.5x the length of his main body AND he had invisible hitboxes extending past them. This made his fair (back then a sideways drill), dair, nair (back then a sex kick), and bair very dangerous. Not only were the hitboxes huge, but his aerials were very quick. Smash DI'ing out of his fair was more difficult than it should've been because of the huge hitbox.
Two spikes + three semi-spikes. Kirby's dair and up B spiked (they still do, but they're cancelable and his up B doesn't combo anymore). To be fair, his up B spike was extremely easy to avoid and once Kirby missed it, he was in for a beating, but still, it was good having something like that to end combos with, if you're near the edge. In addition, his d-smash and fair were semispikes. If you didn't Smash DI his fair correctly, you could be sent down at a 45 degree angle, and his dsmash and dtilt, when tipped, sent foes horizontally.
Huge recovery. But he still has that, although now he's overshadowed by Jiggs, Samus, Peach, and Mewtwo.
Kirby did have some crappy aspects though. His throws were wimpy, his down B was useless, and once his up B was interrupted, he was screwed.