What the Metroid franchise involves, in the most general terms, is a futuristic female bounty hunter exploring alien worlds and gradually acquiring more and more weapons, items, and energy tanks (the series' version of a life bar) while kicking ass and solving some light puzzles. The original Metroid for NES was inspired by the movie Alien, so that should give you a good idea of what you're in for.
I traded some crap in and bought MP3 last night. It is god damned awesome and has the best console first person shooter controls ever. And I truly mean that. It's the closest you can come to keyboard-and-mouse without being keyboard-and-mouse. Don't mistake this for a shooter, though; Metroid Prime games have always been more of a first person action/adventure game than anything else. I'm only about two hours into the game and already I like it more than Metroid Prime 1, though that's more because this game does place a bit more emphasis on action (the first Ridley fight, which isn't a spoiler so shut up, is like the part in Lord of the Rings where Gandalf and the Balrog are falling down that huge pit!!) and much less on the feeling of solitude and creepiness that most other Metroid games go. By that I mean, you actually talk to people and see them in the environments. There will be a few moments where you start to think "man, am I playing Halo??" but the game quickly does away with it before it becomes that.
