Richard William's 'The Animator's Survival Kit' explains everything you'd ever need to know about animated walks.
The first thing I notice about your walk is that the head is kept in the same position, which is incorrect: it bobs up and down. How strong this effect is noticed depends on the type of walk.
And your first and last contact positions are off. The legs stretch in the contact positions (with the back leg just slightly bent), and the feet do not lay flat on the ground in that position.
The middle passing position is incorrect as well: the right foot should rest squarely on the ground, while the other foot/leg passes in a bent uplifted manner.
In essence, your key frames (the first frame, the middle frame, and the last frame) are wrong.