It improved nothing in gameplay terms (besides that neat item selector that appeared if you held the A button, but even then it was better to just pause the game). There were new weapons but nearly every FPS has new weapons.
You have to admit that Perfect Dark's weapons were a lot more interesting. Things like the Laptop Gun and the whatever-the-hell gun that could snipe through walls. (Seriously, what could be more badass than a laptop that turns into a gun that turns into a sentry gun?)

Then there was the multiplayer mode, which added a hell of a lot of options, including bots.

Maybe a better question would be, what did Goldeneye do better than Perfect Dark?