Hello there, seeing as how you awoke this group from it's tomb, I'll give you CC. =)
That arm technique is pretty genius. I wonder why I haven't thought of it yet - it looks wonderful on the figure! Actually, the whole figure looks nice!
About the go kart, while I'm not a builder that makes cars, but it looks awesome! The only thing I'd probably do is to add tiles to the studs on the back of the vehicle. Urr... let's say the top part of the back that needs to be tiled. But like I said, I'm not a cars person. So my information could be inaccurate. If it needs the bumps, then I think you should keep them!
Leg poseability is a must. Even if the fig isn't going to be used outside of the vehicle, it still would allow for some more interesting leg positions inside the cart.
The arm technique is a nice technique, but I'm not sure that I like it as the piece creates the knob at the joint that sticks out.
The greebling at the back is okay, but it's very blocky and un-detailed. It would probably look better if you used some more variety of pieces rather than just the round 1x1's and grill tiles.
The steering is very nice, it reminds me of some of the older technic sets with the gear meshing with the rack at an angle. The hat is also pretty nice, though it would be better with the hole in the round end covered, perhaps with an upside down 1x1 round.
Due to the edges on the slopes, the seat has quite a few bumps on it, it might look better with a an angled bit there.