it looks like you're bending your lower back a bit at the setup, you're definately not as tight as you want to be.
The lockout is very messy, you're using your back to lock out instead of bringing the hips through with the glutes, and it doesn't look like you're holding your back arched in the lockout either. I don't know if it's the weight, or if you just haven't got the form down quite yet.