A proper CRPG comes down to a depth based game world and story line, something that excites you to participate in, something that has great atmosphere and immersion. In my opinion Fable lacks these things. First of all a proper RPG revoles around a very strong source of non-linearity, depth, and open endedness. Aside from the obvious importance of a decent character creation and progression system, multiple solutions and overall approach methods to the completion of quests and overcoming of randomly generated or self imposed situations is extremely necessary, Fable again lacks most of these elements.
Fable's graphics engine as the one major aspect of that game that gets people's attention (which is sad), has weird flaws, the NPC models are overused constantly, the character models are stiff, lack any type of fluid motion capture, have horribly exaggerated bodily proportions, and aren't attractive in any way. Regardless, it's way too linear, in terms of exploration, the game world has annoyingly restrictive terrain (as one other poster mentioned), that prohibits random wandering because of barriers and invisible walls.
The role playing game elements are hard to see, you can only accomplish quests either good or evil, no options for just basic quest solutions that aren't based on being good or bad. The morality system is awful and terribly unbalanced (I shouldn't recieve more evil points for divorce than I do murder, or more evil points for petty theft than unprovoked assault upon an innocent civilian).
The conversation system doesn't exist, in its place are patheticly humorous 'expressions' that are stupid meaningless acts of repetitive sounds and gestures that are supposed to represent an emotion
I personally would give this game a 4.5 out of 10 in the RPG genre, but since it isn't an RPG and is just a linear action game, I'll give it a 5.5-6.5 out of 10. If you want to know why I dislike this game so much, it's simple I waited 2-3+ years for the greatest, most non-linear RPG ever and got an action platformer, everyone go play Fallout, Fallout 2, Arcanum, Diablo, Diablo 2, the Elder Scrolls 3 Morrowind, Vampire the Masquerade Bloodlines, KOTOR/KOTOR 2, or Baldur's Gate, because Fable isn't worth your time. Sorry for the long post.