A short main quest does not quite do justice to the wonderful world in which Fable is set.
Everything about the presentation is wonderful. The graphics are great. The music is thrilling. Things you do leave a permanent impression on the world and you can play the game pretty much any which way you want.
Your character--who goes by a nickname which you can change later in the game if you chose--has his village burned by evil bandits. At the end of this raid, your hero is left alone, his entire family killed. A wandering hero by the name of Maze takes your character under his wing and takes you to the Heroes' Guild where you train and master the martial arts. From there, as you would expect, you quest to become a hero. During this time, you find out many things about your family and the quest naturally turns towards revenge.
The great thing here is, aside from a dozen or so quests you have to complete, there is a good deal of openness in how you play the game. You can be good or evil. You can take wives and buy houses. You can slay villagers for no good reason or protect merchants on quests to sell goods. You can make money by buying and selling goods yourself. You don't really have to even try to beat the game at all to build your character to your liking--though you'll be missing out on a lot of the experience points necessary to upgrade by not completing the main quests.
The problem is what you do have to do is relatively straightforward and short. Despite the open world, the plot is actually quite linear. You always begin with your village being burned and end w/ the confrontation w/ the guy building the world. There are several different endings based on how you play the game but you can't, say, decide when all is said and done that you want to join forces w/ the village burner no matter how evil you've played your character.
I have no problem w/ a linear plot in an RPG in many cases but when it's a game like Fable that really wears its openness on it's sleeve, it just doesn't quite fit the spirit of the game. In this case, it feels to me as if the developers threw the plot in as something of an afterthought and chose to rely on gameplay and character customization alone as motivation to keep playing. Fable's open world is good but it's not so good as to overcome a lack of substance in it's plot. That said, after finishing the game, they let you keep playing and I totally ran around for hours raising hell and having a blast so you can take all this criticism for a grain of salt.
Also, as a side note and service to my fellow gamers, I'd like to mention there is a point late in the game where you can get yourself auto-saved into an unwinnable situation. The game makes it clear when you are entering onto this part of the journey but does not make it clear you'll be unable to stock up on supplies. Make sure to use your character and world saves frequently.