I love looking at games from what amounts to the puberty of video games, there's so many things to talk about. Transitioning to 3D was an awkward time for video games. Many 3D games of the era had what are commonly referred to as "tank controls" -- You turn your character left and right, and then you move forward and backwards. You never performed these actions at the same time, until Super Mario 64. SM64 changed how 3D controls were thought of, thanks in part to the analog stick on the N64 controller.
And that's what really defines Mario 64. It's Super Mario, but in three-deee! Not all parts of two dimensional Mario made the transition, however. The movement has beeen overhauled heavily. Mario can now jump off walls, do ground pounds (a move given originally to Yoshi), triple jumps, backflips. Gone was the grow/shrink mechanic (Mario now has a life bar), gone was the fire flower. Mario levels in the past were traditionally linear affairs, but 3D Mario levels were more playground/sandbox environments. There is no two-player mode, and thus there's no Luigi. Really, M64 redefined what a Mario game was.
That said, Mario's new control scheme feels really smooth. However, it definitely shows its age, especially if you are used to newer Mario titles. Mario has a punch and a kick (the only game in the series to have these), which work fine, but you can still jump on most enemies. For some reason, Mario will not flip around when you go the opposite direction sometimes, which can really screw up some jumps. If you so much as put your toe on a sloped surface you will begin to slide.
That's really where the challenge lies, navigating the environments. As stated previously, the levels are structured sandbox-y. What this means is that instead of getting to the end of the stage, you have to complete a mission to earn a star. By and large these stars (7 per level) can be earned in any order. Sometimes you need to do something special to earn these (e.g. attack all the ghosts, beat a mini-boss), but most of these are behind platforming challenges. Most of the challenge, and hence frustration, will come from falling off of stuff. Most of the levels involve climbing, and one is even suspended over a giant bottomless pit. The satisfaction for completing these challenges is high, though, so I don't hold it against it too much. I can safely say this game is damn fun to play.
The graphics and art aren't too much to write home about, which isn't too surprising considering this was an early N64 title. There are big open areas, which is something that few games of this era could pull off. Many objects are made out of 2D sprites, and thus always face you. The snow effect is especially visually disorienting. As is par for the course of N64 games, the textures are pretty blurry. Even worse, some of them are stretched over large polygons, adding to the blurry factor.
The music is pretty damn catchy, but it's exactly what you'd expect for a Mario game. Many themes are re-used, and many motifs are re-used in differing themes. It serves the game perfectly well, but you probably wouldn't want to listen to it outside of it. Some of the songs can be down right painful to listen to after you've died several times, though. I muted the level Rainbow Ride for this very reason.
I first played Super Mario 64 when I was nine years old, and I was enamored with it. I loved exploring around the stages and completing missions on a whim. It really felt like a whole new world. These days I tend to play this more like, well, a game, instead of just as an experience. This game did a great job of filling both my childhood wonder, and my adult objective focused niches. (Not that I don't like to explore the landscape in games, these days.) I decided to complete this game 100%, and it was quite an experience. I had never done so before, so I got to explore and see new things in a game that I was already pretty familiar. To be honest, it's a little sad knowing I've experienced all there is to see in this game, after so many years of mystery.
If you have not played this game, go get it and play it. Even though it's showing its age, it's still a lot of fun to play. I don't recommend most people go for all 120 stars, though.