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The ending cutsene. After flying a nuclear bomb off the coast of New York and causing it to detonate in the water surrounding the island, Alex lays in pieces on the outskirts of the city. A crow attempts to eat his remains, only to be promptly consumed as Alex's mutilated body rebuilds itself From a Single Cell, proving that he has become Nigh Invulnerable. And he still has things to take care of.
After spending a few missions with a parasite inhibiting his powers, Mercer eventually devours a Hunter packed full of superpower-restoring antibodies. A brief cutscene follows, in which Mercer develops a Blade Below the Shoulder, Organic Body Armor and a Sculpted Physique, and says: "I'm back."
And considering how frustrating it was to basically not be able to do anything while you were a victim of Brought Down to Normal (relatively) you will most likely go "Oh, HELL YES!"
The secret cutscene unlocked by completing all the scientist side-missions.
Karen Parker:"Oh, my God. Oh, my God, he's here. He's in the building!"
The fight with Elizabeth Greene. Times Square, tanks, helicopters, and infantry fighting against the infected, with a Kaiju-sized beast in the center of it all. And Alex is hungry.
How about the final boss fight? The easiest way to win it is to grab 3x your size airplane bombs and throw them at the boss. For that matter, anytime you pick up something bigger then yourself to throw at things.
Cross taking on Mercer in single (well, kinda single) combat and living to tell the tale, by taking advantage of his freaking flashbacks.
The mere fact that Blackwatch soldiers have a melee attack (and it does decent damage, too!). Picture this: you've just seen an unstoppable monster chop up tanks and throw around your buddies like ragdolls. He is now in your face, ready to tear you apart. What do you do? Do you run? Do you hide? Do you scream, beg for mercy? If you're the kind of badass you need to be to join Blackwatch, the answer is none of the above. You instead whack said monster in the face.