I can go on all day about the setting, and the game play, and the cool art style.
while the story is linear, the missions are not. It reminds me of the old Metroid games, the delight of discovering a secret room, or a completely alternate path to your objective.
A ton of detail was put into the little things as well. While most people skip over the optional little E-mails and messages to read, I indulge myself in them, learning about the world I am in, and the people who I am with (or against in some cases!)
I like how you can play the entire game without killing a single soldier (not bosses)
I like how your choices actually have effects on gameplay. I actually have a neat example without any spoilers. At one point you have the option of making a small nuisance go away, but later on it causes you to lose all of your augments during a VERY challenging boss fight (they just happened to leave that part out

).
I like how the immediately obvious option isn't always the best
I love how some bosses are puzzle/conversation/omgwhatisthisgivememore!
I like how the main character tries to sound like Clint Eastwood (don't tell me he isn't!)
I like how your choice in Augs lets you craft your own gameplay, you can go cloak, speed, and sniper rifle to do Hit and Run attacks; stealth, hacking, and CQC to let the enemy's own defenses do your dirty work; and I have even done speed, inventory spaces, and damage reduction to set up elaborate ambushes consisting of grenades and mines.
the choice is yours to make!
Did I forget to mention the awesome variety in missions? Silly me!
while most of the campaign missions are some variety of "go here, avoid/ kill the guards, go here, cut-scene, the side missions offer much more.
on some you get to be a detective, on others a hired gun, the best part is that most of them have 2-3 different outcomes!