Cartridges: Gotta love NO LOAD TIMES and being able to take your game pak with you to a friends and have all your progress come with you even if you forgot your memory pak (I may not be able to dress up my lil bomberman for multiplayer but at least all my progress is saved!)
Controllers x4: The controller was awesome for many reasons; analogue stick, Z-trigger (best placed button ever), it was like an all-terrain controller and the fact that you could plug four of them in (without need of a special adapter ala the NES/SNES/PS1/PS...2!??! are you freaking serious? The PS2 didnt even have four ports? next gen my ass) and have an easy party.
Expansion Ports: I know the port itself is part of the controller but the memory and rumble paks deserve love all on their own. Sure memory cards were done before, and rumble is standard now (unless you bought a PS3 at launch) but back then they were an awesome thing. The rumble pak always gets more attention for obvious reasons so I don't have to say much here, I will however say that the Nintendo 64 Memory Pak is by far my favorite memory card ever for one simple reason:
You plugged them right into the controller. You didn't have to get up and change it out of the system when your card was full, just swap it outta the controller. Were going to a friends house to play some games wasn't a hastle either, all you needed was the controller and you were good, cuz the memory card was in the damn controller already.
With the Gamecube, you don't know how many times my friend would bring Melee over to my house and forgot the memory card with all stages and characters
The N64DD.
D'oh! I would of loved for this to come to the states, I've never wanted anything more than to play me some Mario Artist and be able to import my images into other games... *sigh*