One thing that I preferred from the original film, that I thought was somewhat lacking from this remake, was there was more of an overt focus on the parallel between the undercover cop and the corrupt mole. At the beginning of "Infernal Affairs", when the undercover is 'kicked out' of the academy, the instructer chastises his remaining recruits not to wind up like him, asking them, Would you trade places with him? The corrupt mole - one of the other trainees - says to himself, "I would trade places with him."
And that was a recurring theme in Infernal Affairs. The undercover, who wanted more than anything to end the crime lifestyle and just be a regular police officer, and the corrupt mole, who hated to live a lie as an officer of the law while selling his soul to the crime boss. And at the end of the film, the corrupt mole seems to feel remorse or guilt over what's occurred, because he again thinks back to that moment as a trainee, and thinks again to himself, "I'd trade places with him."
There was some of that with this film, but I thought that whole aspect of the plot didn't get quite as much focus as it should have, when IMO, that is the TRUE core of this entire story. It's Leo and Damon, and how both are being ate up, how both are losing their souls over what the're doing, only from opposite perspectives.