As Captain INSERTNAMEHERE Shepherd, a battle-scarred war hero/heroine, every decision you make will have severe repercussions. You are (or rather, you become) the first human Spectre - an elite intergalactic soldier essentially above the law. Your one remit is to maintain galactic peace whatever the cost. But as with all celebrities, what you say is often as important as what you do. You'll come across a nosy reporter who thinks you're just a shill for the council; lamp her one, and public opinion back on Earth may not be so great. You'll have to keep on the right side of your squad members - during the opening mission on Eden Prime, we were very curt when addressing a young soldier named Ashley Williams, and were surprised to discover that we needed to spend time smoothing things out with her later on.
Later on, you'll need to play diplomat, as distrust grows between human members and alien members, and if your ideology strays too far from theirs, you may find squad members upping sticks entirely. It doesn't help matters that the Council doesn't exactly have your back; humans are a poorly-regarded species within the Alliance, and you'll not be short of people who would like to set you up for a fall. What you say seems to have a very real impact, with chastised team-mates looking at you with big round 'who, me?' eyes. The dialogue trees are excellent. Not once will you find yourself just selecting whatever to progress the storyline. You wouldn't want them to think you were a bastard.
Morality is a complex issue here; you won't sprout crap devil horns like in Fable, but you'll generally find that you get treated as your actions deserve. It's set up for multiple playthroughs (a fact confirmed by some of the game's Achievements) but as engrossing as Mass Effect is, there is a gameplay barrier that, while not enough to ruin the game, might cause you to develop flu-like symptoms if you ever even contemplate playing through it all again: the combat system.
When you're not sitting around gassing with aliens and changing the very course of the universe with your conversational threads, you're starring in a squad-based third-person shooter. And, it must be said, quite an unrefined one. The concept is that this is to shooters as Jade Empire was to action-adventure games, replacing turn-based combat with real-time action, eliminating the age-old RPG malady of the level grind. Certainly it pulls off the RPG integration with a great degree of finesse. Tapping the right bumper pauses play and brings up the weapons menu for you and your squad, while the left bumper brings up what may as well be the 'spells menu'. What's available to you and your squad is dependant on what areas you've concentrated your experience points on (you can ask the game to allocate them automatically, but we'd recommend doing it yourself, if only to get a decent understanding of how everything works).
The skills are essentially age-old RPG conventions with a sci-fi twist; you can defuse barriers or set up your own; perform a weak neural shock to all nearby organic enemies, or provide temporary boosts in armour or firearm skill. Dependant on your class type or the squad members accompanying you, you can even learn and unleash more far out attacks, such as a 'throw' attack that reminds us distinctly of the Force in KOTOR. But don't waste all your XP on offence; decryption and electronic skills are also important, particularly if you're looking to maximise your understanding of the Mass Effect universe.
Bad cover version
Combat is slow-paced and ruthless. And you really are in command, here - if you lose the plot for a second, nine times out of ten, the situation becomes unsalvageable and your entire team's history. If you've dived in clean from Halo 3, they'll be scraping you off the Eden Prime tarmac multiple times before you get to grips with it. These sections are a strange mix between Ghost Recon, Gears of War and (promise us you won't run away) Perfect Dark Zero, and generally they work really well when sparring against the more intelligent races, where cover and tactical positioning is of the utmost importance. Team-mate AI and pathfinding is generally of a high quality, although they're often prone to hogging the premium cover positions, and you get the odd bout of hilarity where your specially-trained warrior gets flummoxed by a simple door.