Ah, this'll be fun :)!
You have alot to work on, your animations are very stop and go, use flow in the future (1), for the rest, your poses aren't really good (2), you also mess up on physics (3) and as a final you're not doing anything challenging and/or new (4).
1: Your movements start, slow down into pose, start over, slow down into pose. I sure as hell hope you're not doing that in real-life, not for fighting, not for grabbing a drink or whatever, it looks weird, it's unefficient, it's boring and makes your stuff look overeased, which imo is a pretty bad thing. Solution: Link movements together, use the speed of one move and keep it going into the next one, that will look better and you won't have to take all this time to ease it properly! So yeah, make a speedy curve in the momentum line instead of him stopping to then go on. It's like walking and standing still after every step you take for no reason at all...
2: Poses, yes, your poses are very unnatural. When he stops with the kick he has his arms folded into his body and both his legs stretched. If you want me to put an example of a better pose, just ask and I'll give you a comparison screenie. For now I'll just tell you to keep in mind you should use poses like you would in real life, or a pro would.
3: Physics, yeah, you lack knowledge in physics... It's common among beginners, but that's what I'm here for, right? Yeah I am. Anyways, when animating in-air physics, the horizontal (< or >) speed should stay the same, but the vertical (\/ or/\) speed should have a constant amount of extra speed downwards, so every frame, make a stickman go 5 pixels less higher than before, or 5 pixels extra speed downwards (just an example).
4: You're generic, animate whatever your sick mind comes up with! I know that would be far more entertaining and it will make you improve far more if you just get an idea and pull it off! If it doesn't turn out how you want at first, just try again and you'll see how it turns out :p. If you can't animate a move you have in mind decently, just ditch it and think of something else.
That's it for now, I hope I'm helping you with this :D!
Medium beginner by the way.
Edit: Oh dag, that's a bump. SOrry, didn't look for the post dates when I was requested to comment here...