Adaptation Expansion: The movie adds scenes where Edward interacts with Elinor in ways not shown in the book, such as helping her to coax youngest sister Margaret out of hiding and discussing their respective future prospects. Not only are many of the scenes hilarious ("What is swabbing, anyway?"), they help give a reason for Elinor falling for Edward where the book never did. The film also gives Margaret some personality traits, notably being a tomboy. She has almost no characterization in the book. The 2008 miniseries takes its cue from this adaptation, and is much the better for it.
In the commentary the creators express profound gratitude that Margaret existed in the book, as they could have her make the "impertinent" remarks that were necessary to move the plot along.
Age Lift: Emma Thompson was 36 at the time the film was released, playing a character who was 19 in the novel. Because it's Emma Thompson, nobody cares. Emma objected to playing Elinor because of her age, but director Ang Lee insisted that she be cast. However, he also raised the age of Elinor to 27 due to the Values Dissonance of a 19-year-old worrying about becoming an Old Maid.
Ascended Extra: The youngest Dashwood sister Margaret, who is barely present for most of the novel, is given a considerably larger role in the film, as well as a more fleshed-out personality (she's a tomboy and something of a geography whiz).
The commentary for the Ang Lee film includes a great amount of thanks for Margaret's presence, as a child could move conversations forward with her lack of concern for politeness and protocol.
Composite Character: Lucy Steele is a combination of the novel's Lucy and her older sister Anne. Anne would fit under Adapted Out, except that Lucy picks up her Idiot Ball task of confiding her secret engagement to Mrs. Dashwood.
Did You Think I Can't Feel?: Said almost word-to-word by Elinor to Marianne. Subverted when Marianne bursts into tears after the speech and Elinor has to comfort her.
Establishing Character Moment: When Edward's arrival is expected, his sister Fanny all but forces the Dashwoods to let him sleep in Margaret's room rather than the guest room, as it offered a better view. When he arrives, however, he insists on taking the guest room and letting Margaret keep her own room.
The carriage ride at the start of the film establishes Fanny's character as unwilling to help her in-laws very succinctly. It also shows just how easily guided John is by his wife's opinions. In one sequence she turns John from giving them a generous allowance to such a pittance that "they'll be much more able to give you something!"
While Sir John Middleton responds succinctly to Margaret's question about what the West Indies are like, Brandon whispers "the air is full of spices" in her ear, foreshadowing his romantic sensibilities that mesh well with Marianne's.
The Ghost: Unlike in the novel, Mrs. Ferrars never actually appears, though she gets mentioned frequently by other characters.
Ill Girl: Marianne, after taking a long walk in a torrential downpour (more believable than the book's version, where she falls deathly ill from moping about in a damp garden). It's much more effective, to the point that some people remember that having happened in the book - Emma Thompson on the DVD mentions having been very flattered when a fan told her that scene (Marianne walking to see Willoughby's house in the rain) was her favourite one in the book, since it meant Thompson had captured Austen's style perfectly.
Ironic Echo: When Marianne falls and twists her ankle, she's carried back to the house by Willoughby. Later on, when she goes out walking despondently and collapses, she's rescued and carried back by Colonel Brandon. Bonus points for both scenes happening in the rain.
The Lady's Favour: Gender-flipped with Edward and Elinor; he gives her his handkerchief when she cries while listening to Marianne play their late father's favorite song, and tells her to keep it. Later, after she knows about his engagement to Lucy, she's seen holding and staring at the token.
Last Name Basis: Colonel Brandon's first name is never used in dialogue, though the letter that accompanies the piano he buys for Marianne reveals that it's Christopher. In the original novel, it was not given at all.
Love at First Note: Colonel Brandon first sees Marianne while she's playing Sir John's pianoforte and singing. He visibly falls in love with her in an instant.
Meet Cute: Marianne meets Willoughby when she slips and falls in the rain and hurts her ankle. He rescues her, brings her back to her home on his horse and carries her inside, like something right out of one of Marianne's favorite stories.
Rescue Romance: Willoughby and Marianne, as mentioned above. Colonel Brandon later rescues her from a rainstorm while she angsts over Willoughby, which sets the stage for their romance as well.
Spot of Tea: Elinor, to an even more ridiculous extent than in the novel. At one point, Mr. and Mrs. Palmer offer her two successive cups of tea within a period of three minutes. In another scene, Elinor drinks the tea she had ordered for Marianne (without Marianne's ever asking for it).