Minecraft
Imagine a world where everything is made up of squares - or more specifically cubes. You are placed within this world, and are free to do whatever it is you want - build things, farm things, kill things, explore or sit around doing nothing. To make your getting what you need to build things easier you must create tools for yourselves, or make armor to help you stay alive longer. Before long, you find yourself delving into deep monster filled dungeons in search of more and more rare materials like diamonds, so that you can create bigger and better things.
Thus is the driving principle behind Minecraft. The version in which I have played is perpetually behind in content updates, understandably so considering how much later to the plate the Xbox version arrived, but the content continually trickles out at no price (outside of the cheap Arcade cost of the game download itself) to keep interesting and new things introduced to the player who might be getting bored with it's rather simplistic design.
How much fun a person will have playing this game is up in a flux - there is no real conceivable point to the game (a newer updated added a boss named "the Ender Dragon" to strive to reach and battle) and so it really boils down to imagination being a direct factor into the perceivable fun to be had. For people that love playing with legos, for instance, the amount of fun to be had in this game is astronomical (especially when the recently added Creative Mode gives you free reign to simple create things from air without needing to first gather materials) as they build and shape the world around them into complex cities, existing logos, or something straight out of their imagination, On the other side of the argument, if fun is directly linked to how much of a plot there is then you will be quite unhappy with what you are given - essentially nothing. Enough of an overview though, lets get a little more detailed.
The mechanics of the game are simple, making it easy to pick up and generally intuitive. To acquire a resource (such as wood) you simply walk up to it and punch ("mine") it. You then take gathered resources to a menu (mapped to another button) that allows you to combine a certain number of different items to make something new - such as a crafting table that opens more crafting options up. Some crafted things like picks or axes will allow you to gather different materials - such as stone - more efficiently or in general (one cannot obtain diamond unless they have a iron pick, for example). Alternatively, one can take non-crafted items and place them down in the environment to make walls for a house, a bridge across water, or simply a barrier to hide from the monsters that spawn in the darkness.
As simple as it seems, this provides for a great amount of depth, especially when latter materials such as redstone becomes available - at which point one can start combining it with switches and torches to make complex devices such as hidden doors and electric mine cart rides. It's this simple complexity that most people will enjoy - the ability to just roam around and create to their hearts content, without being forced to feel like they are missing out on a story by doing so - that also can cause it's downfall. Without the driving force of a story, as soon as someone gets bored of building and creating alone or with friends, the amount of potential starts to dwindle - which isn't the same as saying there is nothing to do, as if you can create it you can do it (to the extent that people have recreated other games such as Connect 4 or Pac Man using the in game mechanics).
Graphically, this game looks good. It's a tricky thing to say, and you'll have to hear me out about it before you get to crazy yelling how I'm blind or something - the game looks as it was intended. Everything exists with that special pixelated feel, while still being very much smooth squares (even the moon and sun are squares) that help the game look the same regardless of the size of the screen. It also helps to give the game a very charming and unique - that many games have started to copy afterwards. The blocks, blocks everywhere design also works very well mechanically in the game, allowing you to generally always have a good idea of what something is just by looking at it as well as making the building things and breaking them down easier to account for easily.
Plot here is only as good as you make it, in the sense that although one doesn't seem to exist you can create one for yourself with friends. You could, for example, create a couple of elaborate towns (with zombie spawners), and have you and your friends load in and pretend that a zombie apocalypse happened, forcing your characters to try and break free of the zombie-city and find a sanctuary. Alternatively, maybe you just build a couple of floating boats, with TnT launching "cannons" and battle it out to see who gets the booty! Yet again, maybe you aren't worried about a plot at all, and are just enjoying the relaxing day of building your blocky dream house to show off to your friends laters. It is entirely limited by your imagination here (mostly), so if you like plots and can't be bothered to create a world in which you could make your own story then you might be disappointed playing this.
The soundtrack really consists of one song that plays on a continual loop. It's very peaceful, which to some might be a bit of a nuisance, although nothing says you don't just load up a playlist onto the xbox (or media player) and use it to over-right the in-game music track. The actual sound effects, in comparison, are remarkably identifiable after having played for a while. When you are roaming around and hear the trademark "hisssss " of a Creeper, for example, many a player will panic and immediately try to figure out how to survive without having all their hard work blown up. After spending who knows how long building things, you get to be attached to it, and the thought of it going up in a puff of green-checkered rage is usually enough to get more emotion out of someone than your average shooter is capable of.
Enemies exist in the game (if you have the mode set to survival), and although they do to some extent act different (a zombie will walk stupidly after you as straight as possible, whereas a skeleton archer will casually try and kill you from afar) it's nothing so elaborate that you need years of playing to figure out. Ambient dangers tend to be the far more dangerous thing - falling into lava, for example, will set you ablaze constantly damaging you until you use water to put the flames out. These can lead to very entertaining moments where you are someone you are playing with will suddenly freak out as they turn a corner and walk smack into a creeper, or accidentally kill themselves when digging straight down only to uncover a pit of lava.
In the end, I can't stress enough that this game isn't for everybody, but that anybody can pick it up and learn to play it. It's a great family game, as you can build and explore together, or just run around causing each other grief as you destroy their fence and let loose their chickens or light their wooden house on fire. If you have a hearty imagination, you can easily find yourself spending hours playing the game without ever once questioning why you did so - for something so simple, it quite frankly is just plain fun.