One of my favourite games to play with movies, is "How I would have directed this differently."
I think Priest could have been a good film. It has a futuristic Western motif, good art direction, an interesting back story and oppressive religious elements, and could have had some fabulous kung fu fight scenes with a bit better editing, more story depth and possibly avoiding the overly alien vampire effects. Seriously, it could have been a much better movie.
It's still watchable, but slightly schlockie, which is too bad. You kind of feel it's a good opportunity wasted.