SPOILER: Elevator death

I still remember my reaction when i first saw the scene. Pure shock. Did not see it coming at all.

in the original (Infernal Affairs), the cop in Trooper Brown's position (the one who confronts them on the roof) turns out to be the other mole and shoots their equivalent of Costigan as they're getting into the elevator before it goes down.
So anyone who watched the original BEFORE watching this one will be expecting the same, and so when the elevator door closes and begins its descent, they'd think "oh, i guess he's not gonna get shot then, they're gonna go with the hollywood ending of the good guy getting his man and being the hero" and then BAM when the elevator gets to the bottom! If Scorsese was going for this, it's quite brilliant because it lulls anyone who'd already watched Infernal Affairs in to such a false sense of security, it means Costigan still getting shot is also a shock to them!
That said, Trooper Brown being the Costello's second undercover would have made for a much better twist, given that he was in the academy with Costigan - Costello could even have pulled some strings or recommended him to Sullivan when he was picking his team.