Fallout 3 takes ambitious steps in every corner, and it does it very well. No wonder why it's the game of the year.
For starters we should explain that Fallout 3 mixes in a very interesting way two basic styles of gameplay that we know very well (or at least should know very well): The first person shooters, and the role playing games, putting the best of the two styles in the game, so that we have a fast paced action shooter, which demands the player to have good reflexes and strategies in the battlefield, and we also can improve some qualities in our character to make him stronger in the battlefield, charismatic in conversations, agile for stealing, sneaking and lock-picking, and smarter for hacking in to terminals, each of those attributes are handsomely rewarding and affect dramatically the way you can approach each objective in your missions or your free roaming through the capital wastelands.
One great add on in Fallout's 3 gameplay is the V.A.T.S. aka the Vault-Tech Assisted Targeting System, in which you can freeze the game, analyze your enemies by body parts and choose which member you wish to shoot, of course paying attention to which members are more likely to be hit, this adds to the game some strategic value because in V.A.T.S. mode you can choose to shoot an enemy's weapon if it's about to break, or shoot a leg so yo get to cripple it and avoid him getting away, or just aim for the head and add more damage to your shots, but be careful you don't have unlimited action points, so you can't use V.A.T.S forever, and you can get crippled as well so don't forget to take some helmet for a ride.
Other RPG's characteristics Fallout 3 has are the large variety of weapons, you have melee weapons, like swords, chainsaws, baseball bats, you also have guns, that can be either small guns or big guns depending if you use both arms or just one when the gun is equipped, laser pistols are also listed, and unarmed weapons, that are weapons used on the fists. Also you can create your very own weapons if you own the necessary schematics and the required items to manufacture it, your weapons get wasted after several use and they all have a condition which must be maintained at high levels to increase the performance of the said weapon.
You should also know if your gonna go out to the wastelands that the nuclear war not only destroyed the whole place, but it also let some spots with high radioactivity levels, and if you absorb to much radioactivity you will get sick, your S.P.E.C.I.A.L. attributes will lower as well as your skills, and eventually you'll loose health. Also there are some nice little items that you can use, this are named Chems, but don't let the name fool you, Chems are actually drugs, and as you know most drugs create addiction over the user, so go easy with this items, they have great effects but you don't wanna get addicted, believe me.
Character customization is very wide in Fallout 3, both physically and attributary, you can choose in a wide variety of clothes which you can get in the shops or from your death foes, get caps (the monetary unit of the game) when selling your recollected stuff, and you even get your own house if you decide to follow a side quest early in the game, which you can use to rest, put your stuff in a safe so it doesn't bother you with extra weight during your campaign.
At the very beginning of the game you edit your character giving him the looks you want for him, later on you'll be told to assign a S.P.E.C.I.A.L. to your man, which means how many points your character will have in Strength Perception Endurance Charisma Intelligence Agility and Luck, this will be your basic attributes, later on you'll choose your skills, which are more specialized attributes, like your ability to lock-pick, speech, hack and your effectiveness with the different kinds of weapons.
To all this you must add the fact of having a great big world to explore, the capital wastelands is really gigantic and offers a lot of environments, ruined cities, people, and beast to get to know. Also there are a lot of well hidden secrets around the wastelands, like 89 unique weapons, some unique outfits, the Quantum Nuka colas, etc.
Fallout 3 is a very ambitious game, it's all about innovation over some game mechanics that already existed, but fallout 3 goes beyond them adding more realism to the wastelands, which become more than a virtual world but a complete new alternate reality inside the game.