— $20.14 — Book
They say that if you like The Lord of the Rings you will not like The Hobbit, and vice versa. The reasoning behind this is pretty obvious. The Hobbit is a children's tale with all of the standard fantasy trappings, as well as the silliness which might attend it, while The Lord of the Rings is definitely adult, a solid mass of mythic storytelling that bears little resemblance to its predecessor.
In fact, the version of the Hobbit that you read today was altered in 1966 so as to be more in line with the Rings series. The encounter between Bilbo and Gollum, that familiar tale that any Tolkien fan knows so well, originally ended with Gollum letting Bilbo go, because he'd won the riddle game. This, of course, lessens the impact of the One Ring considerably, but is more in line with traditional tales.
The major complaint I have heard about The Hobbit in comparison to LOTR is the figure of Gandalf. In The Hobbit, he is much more of a foolish figure than he should be, being worried unnecessarily about certain things. If one gets into the deeper myths that Tolkien built (more on those later), Gandalf seems to be unduly worried about everything, out of proportion to his abilities. He is made to seem human, something Gandalf most certainly is not.
One final note that most people will miss; there are a few asides about the Necromancer in the south of Mirkwood, and Gandalf, at one point, goes off to help deal with him. In other notes of Tolkien (the Simarillion, if I remember right), it is revealed that the Necromancer is an aspect of Sauron, though I don't believe they realize this at the time.