The easiest way to grasp Mass Effect is to imagine Knights of the Old Republic. Strip out the Star Wars licence and replace it with some serious-but-well-crafted original fiction, then remove all the pause-time strategy-style comba. Replace it with competent but unspectacular third-person shooter tactical action, complete with cover system and team-mate ordering. The RPG skills influence the game, in terms of special powers and improved shooting abilities. So, like Deus Ex, the amount your sniper-sight wobbles is based upon your character's ability and the quality of your weapon. And, like Deus Ex, its worth is less in the individual action sequences, and more in the synergy between the RPG development, the action and the frame of a developing story. The results are splendid. Kristan gave it a strong eight, and that's what I'd give it too, so if you're one of those sort of people you can go and get back to comparing PS3 and Xbox 360 screenshots for errant pixels.
There are so many reasons why Mass Effect is so appealing. It's got a typical BioWare plot, which means that there's a mystery that's so well thought out and told that everything clicks into place during the big reveal. Yet it's also a game made by sci-fi fans for those who love sci-fi. There are so many elements borrowed and culled from decades of science fiction and movies, and there are countless moments that will remind you of Star Trek or Aliens or Firefly or some early John Carpenter movie (the electronic score is very Carpenter-esque, too). There's even an effective film grain post-processing effect that makes the game feel like a 70s or 80s sci-fi movie, before digital filmmaking and CGI made everything look and feel too clean.
Mass Effect's greatest strength is the quality of the narrative. BioWare has a track record of producing amazingly engrossing storylines, but never before have you been presented with this level of meaningful interaction. You'll experience difficult and thought-provoking situations, including moral and ethical questions without a clear right or wrong answer. The Shepard character is thus defined by player involvement, sublimely capturing the essence of role-playing.
The trappings of the medium do limit how free you are to pursue different approaches to in-game situations. Your ability to make the more interesting choices is tied directly to how many points you've dropped into the Charm or Intimidate talents, for instance. And as should be expected, different responses to the same issue will often lead to similar, if not exactly identical results. While some of the side-stories could have been explored to greater lengths and with more payoff than a paragraph of text, the main storyline is very strong.