Best RPG plot there was, there is, and there ever will be (maybe).
You didn't think it was possible, did you? An RPG with an original plot? You mean there is not one bad dude at the end I'm supposed to kill/unplug/blow up and/or decapitate? What's the whole point then?! Whose the enemy here?! Figuring that part out was half the fun.
The other half is the icon based combat system (which is my fav) and the non-player generated players that join you (which was still a revolutionary concept back in the 80s).
Nothing, but I wouldn't go back to play it again today.
The Bottom Line
You owe it to yourself to play this classic.