2) Being able to lap opponents and even sometimes second place racer. I loved hitting the computer with a shell right before a jump and screwing them a lap. (Mario Circuit I heart you.)
The best part about that is that the computer controlled racers had infinite feathers, and their use of them seemed to be completely deterministic, i.e. if they did use them, they'd always use them in that same spot. That meant if you placed a banana peel perfectly, every single one of them would jump over it every single time, as well as over the boost and jump markers. The entire pack could be stuck in an endless loop.
Another thing I liked was how speed boosts affected your jump height when you hit the jumps. I loved using a mushroom so I'd be at ludicrous speed and jump clear off the top of the screen. It was possible to do some shortcuts that way, but really, really risky. Which is why it was awesome.
I've been voting for the original the whole time because it was the game I was best at in my prime, and it was a hard game, too. Also, none of the others have felt quite as precise to me. I could snipe with green shells reliably in that game, but in every one since I've felt like the green shells don't get fired quite straight. I just haven't ever developed as good a feel for them as I did in the original in spite of probably logging as many hours in MK64 as in the original. I can list a lot more stuff I miss from the original, like how I prefer the larger turning radius for red shells (that got me killed a lot when first starting MK64), but it's going to turn into nitpicking if I don't stop now.