The original is the greatest game ever created imo.
It wasn't really an FPS, and it wasn't really and RPG; it was a true hybrid. You should play it!
Main + points:
+ Story in both games was told superbly
+ Every action that you take or fail to take seems to have a reaction with NPCs and can determine how your game plays and the events that take place in the story.
+ You're given mission objectives and it is completely up to you how you acheive it. In the original I've played through 6 times and even on the 6th occasion I found ways of doing things that I had never noticed before and areas that I hadn't found.
For example: Do you sneak around the back picking locks? do you hack the computer and reprogram the turrets? do you go in as a 1 man assault team? do you snipe from a distance? do you kill anyone at all or knock them out? do you learn their gaurd routes and plant mines?
+ You have different augmentations that enhance your body and give you abilities such as: temporary invisibility to organics OR robots, strength enhancement, faster speed, silent running, emp shield, spy drones, vision enhancement, health regeneration.
+ The original only forced you to kill 3 people. The rest you could avoid killing if you wanted to.
+ The world was superb, atmospheric and felt alive (even with the old graphics). You could just stop and read information in newspapers, datapads and books. You could get candy and soda from vending machines. You could hack ATM terminals or use them legally. You could buy beer and it'd make you drunk (and people in the bar would respond to your drunkeness). People would wander around the market looking to barter and buy stuff.
+ The game was always set at night (not sure if the new one will do this, but it should do)
+ The games had multiple locations across the globe ranging from the streets of Hong Kong to Area 51 to a large indoor Pyramid with processed air in Cairo.
The games were great because quite simply you had a choice in how you did everything and gave a level of immersion that even current games with their shiny HD graphics have failed to acheive.