Neverwhere is a very traditional fantasy story. The protagonist lives an ordinary, mundane, even boring life, but is content with it. But he (it's traditionally a boy or a man and this is also the case in Neverwhere) is drawn into a world of magic and adventure. He ends up join a quest to oppose great evil. This is at first all against his will. He resents being denied the comforts of his ordinary life and finds this new world disagreeable. But during the course of the story his character grows and adapts to his environment, to the adventure. He learns new things and finds aspects within himself that he is unaware of. He matures and becomes a capable hero. His efforts are crucial for the quest to come to a happy end. The hero then returns home a changed man. The End.
This is a common story in Fantasy fiction. The classic example is of course The Hobbit by Tolkien.
Even one of the major lessons that the hero in Neverwhere learns is pretty traditional: Appearances deceive. Those who at first seem trustworthy may very well betray you and those who seem devious at first might turn to be dependable friends.
Then what new does Gaiman bring to the table, you might ask. Pretty much, actually. What's original in Neverwhere is the setting: The book takes pretty much entirely in London, in the 1990s.
The ordinary life that the protagonist, Richard Mayhew, leaves is that of an office worker. And the magical world that he enters is that of "London Below", a magical city which exists underneath the mundane London, "London Above". This London Below is a world full of magic, but one which exists mainly in sewers and subway tunnels. It's inhabitants are those who fall between the cracks in the world above: those who are lacks homes and employment in the world above take refuge here.
This is a world that is very well fleshed out. It's simply good worldbuilding. The urban setting means that Gaiman can twist familiar fantasy ideas in original and interesting ways. Though, I must make the reservation that I haven't really read any urban fantasy novels before this, so I don't know how original Gaiman really is. But the genre first became popular after the turn of millenium and Neverwhere was first publiced in 1996, so Gaiman was bit of an early bird.
Gaiman uses his setting to make some social criticism. The ordinary life as an office worker in London Above that Richard Mayhew lives is depicted as cold and meaningless. It's inhumane and uncaring. It's highly meaningful that it is an act of kindness and empathy that pulls Richard into London Below. And once he returns to London Above, changed, he can no longer be content with it
The lack of caring shows itself in a major aspect of Gaiman's fantasy world. This mundane world is wholly unaware of magic and that London Below exists. Most importantly, those in London Above can't even see the people who belong to London Below. This becomes almost Kafkaesque for Richard, who once he enters London Below can't be seen by anyone in mundane world, not even his old friends.
There is clear political and moral point made here, because the people of London Below are those who fall between the cracks in London Above. As said, they are people who lost all connection to the world above, such as homes or employment. Gaiman is with this criticizing how society ignores people who are homeless and in extreme poverty.
Gaiman's gender politics are also interesting. The form of fantasy coming-of-age novel that Gaiman is writing here tends to a rather masculine genre. It's often about a boy becoming a man, either figuratively or literally. It's telling that there are no women in The Hobbit.
Gaiman, of course, keeps that bildungsroman form and the main character is still male. But there are two prominent female characters and they are both quite capable. One of them is a princess (figuratively) that Richard saves, but during the course of the book she ends up saving him. Her actions are also crucial in the climax, more so than Richard's.
This is Gaiman's way of lightly updating this type of fantastic coming of age novel for a modern, more feminist world. It's still about the character growth of a man, but this doesn't exclude female characters being important and capable.
All interpretation aside, this is a rather well-written book. As said, the worldbuilding is excellent. The characters are on the whole rather interesting and memorable people. Gaiman is a decent prose stylist. The book is filled with dry and very British humour, in the vein of The Hitchhiker's Guide to the Galaxy, most of which I found rather funny.
Neverwhere is an excellent fantasy novel, that manages to be both traditional and original at the same time.