Honestly, everything in MGS2 seems beefed up and improved over MGS. The controls are tighter and allow for a greater range of movement, enemy AI is a lot smarter and will call in backup if they see something that worries them, and the story becomes much more intricate and fleshed out. MGS2 isn't afraid to kill off a few characters either, creating some interesting tension, as well as establish that not everything from MGS is exactly what it seems. It cranks the weird up a notch as well, which each of the bosses of the terrorist organization you're trying to stop being stranger than the last. There were some unexplained paranormal elements within the first game, but in MGS2, every boss has something utterly surreal about them that makes each one of them memorable and strange. Much of the game is played alongside Solid Snake as well, and being able to see what he can do without playing as him honestly makes him so much more of an interesting character than if you would have played him through the whole game. He just has an effortless cool to him that's instantly addictive.