A classic case of Nintendo doing something perfectly right first time and everyone since making futile attempts to top or even match it (including Nintendo themselves).
Stepping into this 3D world for the first time was a gobsmacking moment. So completely new and yet within minutes everything was second nature. I found myself adapting to the demands of this new gameplay environment without any trouble at all. The whole game seems to have been designed to minimise the kind of problems we still associate with 3D platformers to this day. I never found myself frustrated with the camera, or falling to my death because I couldn't tell which way I was facing.
The world of Super Mario 64 is rich and varied and filled with secrets, but they're the kind of secrets you'll instinctively uncover. Of course you're going to try to catch that rabbit running around the basement, and you're going to try firing yourself all over the levels with the cannons, just to see what's up at the places you can't climb to. You're going to dive down and explore the moat... Super Mario 64 rewards exploration, it knows the kind of things players will try and it almost invariably provides some reward for doing so. Every level branching off from the castle provides a new and different challenge and environment, but my favourites have to be the 'Bowser' levels - classic Super Mario platform gameplay sprung into 3D like a pop-up book. I dream of Nintendo making a new Mario game that consists entirely of levels like that.
Played today it's still completely solid, and its cartoony visuals help an awful lot in that. By keeping things visually simple it outlives the vast majority of games on the N64. Super Mario 64 is one of the fledgling 3D games that will always look good, no matter how much things move on.