Well, first off I want to thank everyone involved with the tracks; you're doing a great job! But unfortunately, I got a couple of issues.
I don't have a big problem with it, but it seems like driving on animated tracks makes the framerate drop by quite a bit. I get around 50 FPS in the Canyon track, but more like 30 FPS when driving in the Old Mine. This has been mentioned by several others, and might be a problem for older/cheap graphics cards.
Second issue is with the Canyon track. When driving uphill, the kart seems to not only slow down because of the elevation, but maybe it touches the ground with the chassis as well. Not sure on this one though; it might be that I'm just being a jerk and that it's only because of the elevation itself. The kart shadow gets cut though, but that's not an important issue.
This is mostly just nitpicking, so don't feel too alarmed by this.
Keep up the good work.