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.
Enforced Method Acting: On the first day of shooting, Streep went up to a nervous Hathaway and told her, "I think you're perfect for the role and I'm so happy we're going to be working on this together. Keep this in mind, because this is the last nice thing you'll hear from me."
Family-Unfriendly Aesop: 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: This movie tries to pass off Anne Hathaway as unattractive at the beginning, though it was less that she was meant to seem unattractive and more that it was intended as a comment on the ridiculous standards of the fashion industry.
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.