*chuckles* Cool pic. ;3

I see a few things. The tail for one comes out of the spine usually, so depending on how the character is, it'll either come out just above the butt or lead from the small of the back - yours is a bit too think and unsure of where it's coming from. I also suggest you make a part of it flat to accentuate the fact that he's laying on a flat surface - I'm assuming a yoga mat.
As for the arm, if you look at your own the elbow is always at the waist and the hand is always at mid-thigh - here, your upper arm is far too short and the arm in the back that's holding the legs the elbow needs to go forward more towards the hand just a little.

One last thing is if the tail is one that comes from the small of the back, the kind that's kind of like a continuation of the body (think of Palkia from Pokemon) then the legs wouldn't be able to come flat together as if it was a tail that came from under the butt as a continuation of the spinal cord (like cats, or most canines)

Thank you for the crits! The tail, especially, isn't something I'd noticed--I'll definitely play around with that a bit more. :)