Walt Disney became determined to cast Julie Andrews as Mary Poppins after seeing her on Broadway in "Camelot". Since she was pregnant when shooting was to begin, Disney held up production until her daughter was born. He also arranged for her and her then-husband Tony Walton to be given a personal tour of Disneyland, and made Walton costume-designer for the movie (which would garner Walton an Academy Award nomination). Andrews herself, of course, won Best Actress for her performance - one of 5 Academy Award wins and 13 nominations for the film.
The movie opens with a performance by the very English street performer/chimney sweep Bert (played improbably by the very not English Dick Van Dyke, who got a Golden Globe nomination for the role). The conceit of the film is that the audience is among those watching Bert sing and dance at the very beginning, and we request that he take us to 17 Cherry Tree Lane.
17 Cherry Tree Lane is home to one Mr. George W. Banks, played by David Tomlinson. Banks is the epitome of a snotty, English patriarch:
King Edward's on the throne it's the age of men
I'm the lord of my castle the sovereign, the liege
I treat my subjects, servants children, wife with a firm but gentle hand, noblesse oblige
His wife and children, of course, suffer the consequences of his obsessively self-aware, genteel properness. His wife is Winifred Banks, played to a tee by Glynis Johns. She is ostensibly an egalitarian suffragette - but she has the maid pack spoiled eggs for her to throw at the Prime Minister, insists that she "would... muddle the whole thing" whenever a decision about the house has to be made, and constantly praises her husband's transparently hopeless cleverness. His children live in fear of him, and take out their frustrations on one nanny after another.
Into this situation floats Mary Poppins, who responds to a call placed by the children themselves (Karen Dotrice playing Jane Banks and Matthew Garber playing Michael Banks). She imposes herself on the Banks household, effectively hiring herself to nanny the children. Over the course of the movie, she'll attempt to bring joy into the children's lives and bring the family together.
There are at least three joys to this movie:
(1) The editing and effects. The film won awards for Special Visual Effects and for Editing. Though the movie is probably the best live action Disney movie ever made, it is the combination of live action and animation that marks the movie's uniqueness. The animation is almost naive - certainly it looks antiquated by any modern standards. But in the context of a magical yet proper nanny rolling her eyes and jumping with children into a chalk drawing, it strikes just the right effect. Plus, talking animals.
(2) Julie Andrews as a proper-but-not-too-proper nanny. And I think we all know what I'm talking about. Her voice. Which brings us to...
(3) The music. Richard and Robert Sherman won Academy Awards for Best Musical Score and Best Original Song. The song that won for Best Original Song was "Chim Chim Cher-e," but Andrews is at her best in the haunting "Feed the Birds (Tuppence a Bag)" (hat song, which was Disney's favorite from the score, sets up a withering critique of fashionable, upper-class neglect disregard for the homeless).
There are some other interesting innuendos and subtleties: Andrews makes it very clear that she knows that she's "practically perfect in every way" - and a minute and a half she twice openly admires herself in mirrors of various sizes (although really, who can blame her - and Mary Poppins's vanity was actually toned down from the books). Also toned down from the books are overt hints of a romance between Andrews and Van Dyke.
The film has been the subject of several articles that find in the plot progressive to radical themes - advocacies of suffrage, critiques of bombastic militarism, ridicules of capitalism ("When gazing at a graph that shows the profits up/Their little cup of joy should overflow"), etc. Critics counter that by the end, all of those tensions are resolved into a very conservative "family first" message. Of course, the film actually ends up somewhere in the middle: lambasting the facile hypocrisy of hyper-aware bourgeois pretensions while emphasizing that some social cohesion and many 'old fashioned family values' are sentiments worth preserving. Come for Julie Andrews, stay for the plot.