Metroid Prime was the first game in the series to break from the 3rd person view and give the player first-person control of Samus Aran.
It does a pretty good job of pulling off the Metroidvania style of gameplay in a 3D world (instead of a sidescrolling 2D world), but IIRC it makes some changes to traditional FPS gameplay.
The main addition is the ability to lock-on to enemies; this gives the gameplay the run and gun feel of the 2D Metroid games, but also makes it less difficult than other FPS games (circle-strafing with lock-on targeting is a joke).
One of the cooler additions is the multiple visors; in the ice levels, Samus finds a thermal visor, letting her more easily see certain machines and enemies. Later in the game, she finds an x-ray visor, which lets her see through most solid objects and detect invisible enemies.