The cartwheel to back flip was awesome!
The only thing I saw was that the back was bent at and unrealistic angle. Can you bend your upper back like that.
See you at the hospital my friend.
Stiffness was also evident, but there's some of that in every Beginners animation. Still work on i by moving every joint each frame. Don't forget about other limbs as well. I see that while flipping, one of the arms just hang there. Do something crafty with it, pull it in, or do a skateboard flip thing and put it on the ground as a balance point or something!
At the end when you completed the flip, you defied physics and didn't carry the momentum out, so when you came to land the flip, it was moving like this:
-----/-----/---/--/-
That didn't look right. To fix this, anchor the feet in the ground, and the momentum with throw his back, but if the feet were anchored, it will stabilize the momentum, so it can easily stop.
Maybe to make it longer, add an extra bunny hop when he lands the flip to make it more realistic.