The Scrappy: Lily in the books. Although she and Andy have been best friends since they were children, Lily rapidly starts to lose control of her life, her academics and her drinking problem. As one troper has pointed out, Andy isn't Lily's mother even if she insists on being her best friend and as her emergency contact. If not for Lily drunk driving and ending up in a coma, Andy wouldn't have been put in an impossible situation where her family and Alex expected her to come home from Paris to check on Lily at the risk of getting fired.
Alternate Aesop Interpretation: One could interpret the movie as a big Break the Haughty for Andy. At the start she turns up for a job interview knowing nothing about the magazine she's applying for or the woman she could possibly be employed by. She seems to view fashion as being beneath her, and the other girls as being ridiculously silly for indulging Miranda. Throughout the course of the story, she learns to take fashion seriously and her makeover is a sign of her applying herself to be successful at her job and understanding what she's doing - which any good writer needs to know.
Awesome Ego: She doesn't drone on about how great she is, but everything about Miranda's conduct clearly shows how firmly she believes she is the centre of the universe. In the fashion world, she's absolutely right.
Broken Base: With regards to the depiction of the fashion industry. Several industry figures called it a Cliché Storm that seemed to represent more what an outsider thought fashion was like. Others however praised the film for getting the politics and "sucking up" of the industry spot on.
Cliché Storm: Many pointed this out about the film, notably that it indulges several cliches about usual depictions of the fashion industry - all the workers being weight-obsessed, people being able to afford expensive clothes despite being paid minimum wage, and presenting buying designer brands as crossing the Moral Event Horizon.
Designated Villain: The movie tries to frame Christian as being a dick for knowing about the plan to replace Miranda and the whole thing as a Moral Event Horizon for the character. Andy's "I'm not your baby" is intended as a Shut Up, Hannibal! moment, which some feel is a bit overblown. The only genuinely dickish thing he does is kiss Andy while he knows she has a boyfriend, which ironically the movie blames Andy for.
Ensemble Darkhorse: Despite Anne Hathaway being the lead, most of the praise for the film went to Emily Blunt for her funny portrayal of Emily.
Specifically, Ambition Is Evil. A popular criticism of the movie is that Andy is painted as having gone over to the dark side for having somewhat less time to spend with her boyfriend, family and friends, who are all constantly giving her shit for having a job they don't approve of and implying that she's probably bad at it anyway since they never knew her to be interested in fashion. And in spite of taking this approach, it also passes on the chance to do a Beautiful All Along — Andy gives away most of her designer stuff at the end, but she remains the thinner, sleek-haired, flatteringly-made-up, well-dressed woman she learned to be working at Runway.
It is also worth noting that a key part of Andy's arc has to do with her relationship with her boyfriend, which gets rocky because he doesn't like the ways her job is "changing her" (it doesn't make sense in context either), or that she works such long hours. At one point we see Miranda and her husband arguing bitterly, evidently for similar reasons; and at the end of the film Miranda reveals that her husband has asked her for a divorce. Soon after this, Andy leaves her job and Miranda, tearfully apologizes to Nate and tells him that he was "right about everything".
While the movie does make clear that Miranda's life isn't perfect by any means, it nonetheless seems to provide the aesop of "So long as you make yourself indispensable, you can treat anyone else horribly, up to, and including, being justified in crushing other people just to advance/protect yourself". That is, the underlying message seems to be that it's OK to be a raging asshole so long as you are good at what you do.
Hollywood Homely: Parodied and used to show some of the ridiculous standards of the fashion industry - since Anne Hathaway is described as an ugly fat girl by Miranda. It's clearly used to show Miranda's Insane Troll Logic.
Jerkass Woobie: Miranda's daughters, if you stop and think about it. They're clearly spoilt by her and they direct Andy upstairs knowing Miranda will be furious with her, seemingly For the Lulz, but though Miranda obviously loves them very much, she rarely spends time with them thanks to the magazine coming first. The few times we see them or hear them mentioned besides the scene at Miranda's house, they're always being escorted by Miranda's staff and have had to go through not only several stepfathers, but every newspaper calling their mother an Ice Queen for driving another one away.
Les Yay / Foe Yay: Let's just say that what fandom the book/movie has centers around shipping Miranda/Andy. Also, the movie is crawling with subtext; Meryl Streep's performance was extremely seductive.
Strawman Has a Point: Both Nigel and Miranda tell Andy off in her early days. They do have a point, as she seems to view fashion as being beneath her, which is a tad hypocritical considering she accepted the job. And in this case, knowing about the industry you're working in is kind of important, even if Andy is only an assistant.
Unfortunate Implications: Here about the movie implying that it was impossible for a woman to be successful in both business and relationships - since Miranda is shown to be Married to the Job and neglects her family, with the movie implying Andy is going the same route as her career progresses.
Unintentionally Unsympathetic: Andy's friends are rather nasty, many thinking their attitude of throwing Andy's phone around while her boss is calling is way out of line. Lily blames Andy for Christian kissing her, not allowing Andy a chance to explain the situation. And Nate complains about Andy working so late as if she has some kind of control over it.
The Woobie: Emily isn't the nicest person, but she certainly doesn't deserve the screw job she gets from the universe.