The gang at Play magazine got to play Metroid 3 a little while back. As they were no doubt obliged to do, they've written about their experiences, thus letting us all know some of the things we can expect from the game when it turns up. It all sounds a bit promising:
Wii Remote controls work better than the old dual-thumbstick ones.
It's got beam stacking, ala Super Metroid.
The endless Metroid back-tracking makes (apparently) more sense.
It's got larger environments, better textures and BLOOM LIGHTING.
Play believe, in their own opinion, that it could well be the "biggest revolution in FPS gaming since Halo".

Let's get this bloom lighting party started like it's 2005! More, slightly-spoilerish, less boastful info over at Go Nintendo. Luke Plunkett
Metroid Prime 3: Corruption details [Go Nintendo]