You play the role of Shepherd, a military commander who is given the opportunity to be the first (sort of) human selected to be a member of what amounts to the galaxy's CIA unit. After a few missions to prove yourself, you get the job and then you are free in many ways to do what you want.
The story is by far one of ME's greatest achievements. Unfortunately, this game was released near Bioshock which tells a more detailed and compelling story. I am of the people who think Bioshock's story is nothing more than a minor rehash of System Shock 2 almost to the letter. Aside from that, this story works, and it shows that, with just a little effort, a game company can really deliver a meaningful story experience. In many ways, Mass Effect's story is quite unique.

You'll notice right away that the graphics are top notch. The environments are detailed nicely, as are the characters. Some of the cutscenes are downright gorgeous. But pay attention, and you'll notice something. Maybe it's your TV. Everything seems a bit grainy. Then, go into graphics settings and switch off 'Film Grain'. Wow, what a difference. It's like Bioware gave me a pair of dirty sunglasses to play the game with. Take them off, and the beauty is even more apparent.
I have no idea why this setting is defaulted. It takes away from the graphics. I switched it on and off a few times. Maybe they were going for a noir-ish look, but it just doesn't work. At least for me.