Half of what I like is director Derek Yee's style, which has the feel of a documentary instead of a movie: for all the car chases I've seen in my life, this the first film I can remember that includes a scene in which the cops and paramedics are actually dealing with the aftermath. All the violence feels more real - not that that's something I enjoy seeing, but it treats the characters as human beings. There is a phenomenal scene in which an older cop tries to get through to a rookie that 'bad' people are still people.
The other half of what I found so fascinating is what I would call the 'morality play', as the cops progressively transgress the rules, while Lai Fu (the hit man), struggles upwards towards a kind of redemption. Only a young Daniel Wu (or a young Matt Damon) could pull off this 'hit man with a heart of gold' character, but it's beautiful. There was A LOT in the middle of the movie that reminded me of the first Jason Bourne movie, although Yee's vision is darker and won't allow his character to escape. I got totally sucked in. Loved it, even if the end broke my heart.