Fighting is the only part of the game that I feel has a real system and challenge to it. At least, until you master countering and unlock a lot of moves. Buffed you're unstoppable, but most of the time you do need to chain together combos, watch out for grapples and weapons, keep an eye on enemy types and mix up your moves appropriately.
Everything else is extremely easy. Free running sequences are so scripted it's impossible to lose unless you actively try. Driving combat is a breeze due to the giant hit boxes on wheels thus easy one-hit-kills. Combat is simple thanks to the generous aim assist and mastery of headshots. Slow mo dive over cover, chain headshots, and everybody will die in a second or two.
Actually, I'd say that is my only real big criticism of Sleeping Dogs. I do feel empowered and like an unstoppable badass, and that's great, but there's no real challenge to anything other than fighting. I don't mind because it's so much fun anyway, and by doing so much I don't expect individual components to be super fleshed out. But if a sequel were to happen I'd really like UFG to build off what they've got for more challenging scenarios. Less scripted free running missions, extended driving sequences (eg: unarmed, forced to perform multiple action hijacking to escape unarmed opponents wrecking your cars), less forgiving shoot-outs, etc.