I have a couple of criticisms of your serve.
First, you have the unfortunate straight arm problem in your windup that Nadal occasionally has. Because you straighten out your hitting arm behind you, you have to wait for it to bend into a decent trophy position to complete your swing. Watch all the best pros, they have at least a 90 degree bend in the hitting arm at the trophy position.
Second, you are a little stiff from the trophy position on. It isn't terrible, and you seem to have decent hip rotation, I just think if you could get it going a little forward and up it would give you more power.
Good luck and keep practicing.