Updated November 14th: read my full review of Fable II here

Sum up:
Like Fable, Fable II is a real-time (not turn-based) RPG, and takes place 500 years after the first, with a large explorable overworld (only this one is 10 times larger, and there won't be any "invisible fences"; if you see it, you can go there), and dynamic gameplay; every action decide whether you're good or bad.

There is a plethora of new features, like the ability to be male or female (you can get pregnant if you choose the latter); the cutscenes will now be almost entirely in real-time (you can move your character around during them); you can buy up and manage entire cities; perform jobs in the form of multiple mini-games; there's an extensive online and offline co-op mode; and there is even a sidekick in the form of a very realistic and lovable dog.

Recommended age:
The ESRB rated this game "M' for Mature, listing "Blood," "Language," "Sexual Content," "Use of Alcohol," and "Violence." The "Blood" puffs in light amount when you or an enemy is hit, and the effect is actually more mild than the last game (which was moderate, not gratuitous), and you're still able to behead enemies (but it's hard to pull off). The "Language" is light, and consists of a few uses of d*mn, hell, bastard and a**.

The "Sexual Content" is the most extreme, and consists of being able to marry multiple partners (bigamy), get married to both or either gender (bisexual/gay), you can have sex (although it isn't shown), you can get STDs, females can have children, you can divorce and/or abuse your spouse, there's prostitutes, there are some suggestive gestures you can perform (like the finger and pelvic thrust).

The "Use of Alcohol" consists of being able to drink beer at any time, and as much as you wish (eventually making you throw up). And the "Violence" consists of the main character killing many human and mythical enemies with multiple weapons like swords, guns, crossbows, magic (fire-balls/ice, etc.), axes, etc. This game's (optional) sexual content is quite a bit worse than the last (I gave the first Fable a 17+), and I'd definitely recommend this game for ages 17+.