RPG-lite ... better for kids or casual players than veteran RPG players.
In terms of little touches, Fable is quite fantastic. Day/night cycles, NPC schedules, variable in-game economy, customization through haircuts and tattoos, stylish armor and weapon design ... they've got all the bullet points you'd expect from a modern RPG. The game actually shares a lot in common with World of Warcraft -- playful character design, lots of silly dances and expressions and humor, colorful environments, a streamlined user interface, and mostly straight-forward questing.
Like WoW, Fable is also easy to learn and get into. Quest objectives are clearly shown on your mini-map, solutions are never too tough to figure out, and the game is generally easy to complete from start to finish.
On the downside, however, it means the game never really gets its hooks into you. The combat is simple fun but repetitive -- even moreso when you consider you'll be facing the same half-dozen variety of enemies throughout the entire game. There are a few boss battles that throw minor wrinkles into the formula, but for the most part, the fights you see at the beginning of the game are basically the fights you'll be dealing with for the next 10 to 15 hours.
The story is also standard fantasy fare. While the voice-acting is quite good, the actual plot doesn't stand out from anything you've played countless times before. The game world is also fairly shallow. There's no real overarching mysteries or lore than will command your attention. There's plenty of slightly clever quests and choices. Do you choose to run a bordello or turn it into a women's shelter? Do you choose to save a kid being picked on, or pick on him yourself? Overall, it's always a binary, saint-or-sinner, black-or-white morality system which gives the illusion of "choice" but, like so many other developers that do this (cough Bioware cough), your only choice is to be total nice guy or a total jerk.
At the end of Fable, you can look back on 15-some hours of generally pleasant RPG adventuring. But you can't help but feel a little unsatisfied. There are far better games in this genre for veteran gamers, but because Fable is so pretty and polished, it's easy to see why it's so popular.