First of all i'd say your animation is to slow for a running animation.
i think it would be rather at least 2 times faster.
Then, your character doesn't bounce at all. Look at the head, it stays still during all the animation, it doesn't move at all.
The head should go up ad down on a vertical axis following the bounce of the character while he's runnging.
Also since your character's body doesn't move but the feet are moving horizontally sticking to the ground, the legs' lengh is growing.
Moreover when the character is moving a leg backward the animation is smooth but when it's moving forward you can see that it lacks something. maybe one more frame, the amplitude between the positions of the leg of frames 7 and 8 is too large.
Try to focus on local parts of your animation trying to not to see other parts, you may see the problems better.
Anyway, i think pictures speak better than words so i found this link for you :
http://www.idleworm.com/how/anm/02w/walk1.shtmli hope it will help you to improve your animation