It's the presentation that sells Metroid Prime. The visuals and audio design. The way the environmental design really tries to create a believable space. The details of Samus' helmet display and how it reacts to the environment around her. The ways in which the arm cannon will open and extended when using different weapon types. All together it creates an incredible sense of immersion and atmosphere.

Scanning the environment and enemies for details and clues. Frantically switching visors and beams to take on different enemies. Exploring the and rooms and corridors of Tallon IV.

The biggest problem of Metroid Prime is something that the series often faces. The enemies just won't stay dead. Enemies often respawn after you've moved just a few rooms away. Meaning that if you ever make a wrong move and have to turn around, you'll come face to face with a gang of combatants you killed just a few minutes ago. It makes exploration and backtracking more tedious than it should be and makes combat something you feel like you should avoid if possible.