They open a door and enter a world.
Lewis' writing here is geared strongly towards young children. He seems to delight in breaking the barrier between narrator and reader, telling the story as a storyteller would a gathering of kids. The style is effective in that it FEELS like a children's book, and in a good way; it's a great pleasure to read this out loud. While there are moments, however rare, when you almost feel talked down to reading as an adult, by and large Lewis succeeds in crafting a story that reads like a timeless fairy tale suitable for any reader who likes adventure, of any age. This also means "Wardrobe" may be the book in this series toughest to read for adults, if only because it has the most overly childlike-nature of the seven Narnia books.
The plot itself is simple, with few (if any) complexities and a fast resolution. The entire thing can easily be read in one lazy afternoon.
And then there is Aslan, the lion named in the title. This beast is at the center of the entire series, even moreso than the children who find themselves entering Narnia. What stands out as an adult (but is easily missed as a child) is the strong, overt Christian allegory. It is here in spades, right down to a faux crucifixion.
For many readers tuned into today's sensitivity to religion, or even offended by overt displays of religion, "The Lion, The Witch, and The Wardrobe" will simply be too heavy-handed in its religious allegory for adult readers. Not simply because of the Christian-influenced themes, but because Lewis simply beats you over the head with it. Every mention of Aslan and every scene with the lion gushes with an almost ferocious (and often uncomfortable) feeling that screams to the reader, "Rejoice!"
But to suggest that this detracts from the fun and wonder of the tale would be unfair. It DOESN'T. Only those sensitive to religion and the fervently anti-religious will have a problem with these aspects of the story. Others will just enjoy the ride.
As a children's tale "Wardrobe" is a fun romp through a mythic land, just the kind of wish fulfillment fantasy-loving children enjoy. And for that, I admire this book. Kids will love it - and so will adults. It's not the best of the Narnia works by any means, but it IS the first that should be read because it's the one that started it all. And as a classic of the genre this should be read by all, no matter one's religious beliefs.