Since this an action rpg, combat stands out as the most important part. It's third person real time with a pausing mechanism where you can choose which of your abilities to use and switch your guns out. You have yourself and two companions out of a total of six on the battlefield. You fully control yourself and may take cover, lean out and shoot, or be a total dumbass and run right in front of the enemy shooting. You also do a melee attack with your gun if you are close enough to the enemy which does a bunch of damage(there is a talent you can raise to do more). There is four different gun types, three combat abilities, four tech abilities, and six biotic(force) abilities. Everything is useful enough, some things better then others. This is balanced out with the recharge time on the abilities, the less useful ones return in a few seconds and the most useful ones taking about a minute to return.
Back to your party members, you do not have direct control of them. You can send them to certain locations or call them back with the D-pad but I found that they were usually smart enough that I didn't have to tell them what to do. When you are in the puase menus(called up with the RB and the LB) you can tell them which power of there's to use. You also have full control of there equipment. On the equipment screen, you can quickly change out upgrades on your weapons and armor that give bonuses to accuracy, damage, damage against organics and a bunch of fun stuff like that. The only complaint I have about this is that you don't see any of the numbers change on the weapons and I often forgot what upgrade I had on my four weapons, grenade launcher, and armor. If they just placed a second number next to the first, that would make this alot more fun.
Overall, combat is probably the most fun I had with this game. It's better then most action rpg's, and definitely the best bioware has ever done.
Choices a plenty in this game, it's just missing the other part of the equation, consequences. The only thing these choices ever reward is paragon(light) or renegade(Dark) points depending on how you handled it. But even that doesn't effect much, except something at the end of the game. There is alot of big choices along the road of the main quest that gives you the illusion that you are changing something, at least for the ending. For example, and this is a spoiler, you get a choice of whether to kill or not to kill this bug like race called the rachni. You have the final queen Rachni alive in a tank, and you can either open it or send the poison in. The Rachni caused a bunch of trouble in the universe before, but this rachni says that is all over. It was set up as a very important choice, but nothing. Nada. It's not brought up again. The have it set up so perfectly, so my hope is that it does matter in ME2 but I somehow doubt judging this game.
The sidequests are very interesting in this respect: You have two speech talents, Charm and intimidate, that will change the conclusion of each sidequest if you have enough points spent in them. It usually leads to less combat but sometimes you learn something interesting you wouldn't have otherwise. It's very fun, and probably the second best part of Mass Effect.
At the beginning of the game you have a choice between six classes that gives you eight of the 24(I think) talents. Three pure classes(combat, tech, biotics) and three mixes. Each class has it's own talent tied to the class(Soldier has the "Soldier" talent). They also have the two speech talents and get Spectre Training once you become a spectre. You get three, two, or one talent points each level up and each talent goes up 12 levels. Each level might give you a special ability or unlock a locked talent for your class or just improve the talent. It's overall a simple system, but a pretty interesting one as well. It allows some variety within the classes, and everytime you level up you have a big choice of whether you should raise your combat talents or your speech talents. I haven't found a weak talent, so it seems well balanced as well.
Setting and story:
Your regular sci-fi setting. There is about ten different races, and they all have there quirks and interests. They are still a little bit pidgeoned holed and humans still seem to have the most well difined personalities and variety in quirks and interests. For example, you don't ever meet a skinny Krogan or a Krogan that doesn't like the feel of combat, for example. It's fine, but not great. The story suffers from bioware revealing too much before it came out. I knew pretty much everything up until the final mission, just from the info bioware released before release. It's pretty good, and their is no stupid twist thrown in, I just hope they don't reveal so much of the story for ME2. I would have liked it alot more if they didn't.
If you are in the mood for an action rpg and have an Xbox 360, go ahead and buy it. It's probably the best game out for the 360 right now and probably for awhile. If you are looking for an amazing rpg that will blow your mind, then lower your expectations. It's not amazing, but it's OK. And that's ok...