One of the funniest lines from the book that made it into the movie:
Peter: Girls can't carry a map in their heads.
Lucy: That's because our heads actually have something in them.
In the book, the transition from Earth to Narnia is a highly uncomfortable "pulling" sensation as the Horn yanks them into Narnia. Once they find out that the Horn was responsible, Edmund compares it to summoning a genie and says, "Now we know what it feels like for the Djinn! It's a bit uncomfortable to know that wecan be whistled for like that."
Trumpkin calls the Pevensies "dear little friends" because he wasn't expecting them to be children. They point out how hypocritical it is for a dwarf to call them little and start calling him the Dear Little Friend instead, to which he immediately protests. Then they abbreviate it to D.L.F., and the narration notes that the kids eventually forgot what D.L.F. even stands for.
BBC Version
The first episode opens with the Pevensies at the station - sprinting to find a free bench to sit on. The force of it knocks another girl completely off the bench. Susan then taunts Lucy for her unladylike behaviour.
Susan: Lucy, really. Who would ever believe that you were once a Queen of Narnia?
Lucy: You jolly well ran too, Queen Susan.
Film
The Pevensies rescue Trumpkin from a group of Telmarines. Susan shouts "drop him" and one of the Telmarine soldiers quite literally drops into the water, and Susan swims and rescues him.
Trumpkin: Drop him?!
Susan: A simple thank you would suffice.
The quickest way the Pevensies know to get to Aslan's How (formerly the Stone Table) is to cross a river called the Rush. When they finally find it, after 1,000 years in Narnia Time, the peaceful little river that used to be no trouble to ford is now at the bottom of a deep gorge.
Susan: See, over time, water erodes the Earth's soil, carving deep into the...
Peter: Oh, shut up.
It takes them a while to figure out where they're going:
When the hag and the werewolf try to resurrect the White Witch, Edmund stops them by smashing the block of ice she appears in.
Edmund:[to Peter] I know. You had it sorted.
When they're at the top of a cliff, we get this exchange.
Edmund: Is there a way down?
Trumpkin: Yeah. Falling.
Lucy finding one of her old dresses from when she was Queen of Narnia.
Lucy: I was so tall.
Susan: Well... you were older then.
Edmund: As opposed to hundreds of years later, when you're younger.
Lucy starts referring to Trumpkin as the 'DLF' (dear little friend). He quips that the name isn't at all patronising.
Reepicheep's desperate justification for wanting his tail back, with Eddie Izzard somehow managing to make the book's original dialogue sound like something straight out of her stand-up material.
Edmund casually (and skillfully) manipulating Miraz into the duel with Caspian is even better than in the book, combined with a hefty dose of awesome. "So you're bravely refusing to fight a swordsman half your age?"