The easiest way to describe Metroid Prime is this: You know how Mario 64 was so succesfuly because it changed the premise from crossing a 2D plane towards a goal to a series minor tasks in a major 3D environment? Metroid, however, didn't need to change a thing. The greatest strength of Metroid was always exploration, and the addition of a third dimension only enhanced that.
If you enjoyed a 2D Metroid, you'll enjoy a 3D Metroid. The idea of switching to a first-person perspective may put you off, but once you start playing it immediatelly becomes clear that putting the camera inside Samus Aran's helmet only adds to the sense of immersion. Ambient sounds combine with a camera that reacts naturally to its surroundings, doing a better job of making the player forget they're sitting in the living room and focus completely on the environment of the alien planet they're exploring.
Gush-tapping demands at least one complaint, and for the Metroid Prime series that complaint belongs with the Gamecube controls. However, with the entire Prime series having been re-released with flawless Wiimote precision controls, that complaint is completely eliminiated. Beyond that, Metroid Prime manages to convey not only the greatest features of the 2D Metriods but also the minor annoyances. Yes, there's lots of backtracking, yes, you may get lost, and yes you'll waste plenty of spare time hunting down those last few missile upgrades preventing you from reaching 100% completion.
Metroid Prime is Metriod in 3D. Undoubtedly the textbook example of the transition done right.
Score: Almost perfect.