These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
George Sanders himself sees Eve as a "the closest thing to a heroine" in the story and Margo as immensely dislikable and unrelatable. This is probably why Showgirls which is this except with Vegas showgirls - made the 'Eve' the main character.
Could Karen's sabotage of Margo's lift to the train station (making her miss her performance) be just her trying to give Eve a chance - or a moment of frustration wanting to knock Margo down a peg after weeks of her being unbearable.
At one point, Margo quotes from Julius Caesar ("the evil that men do", though she can't remember what comes after that). Three years later, Joseph L. Mankiewicz went on to direct an adaptation of Julius Caesar.
Margo refers to her maid Bertie as a "fifth rate vaudeville star". Fast forward to What Ever Happened to Baby Jane? where Bette Davis would play a formerly famous vaudeville child star, in the shadow of another woman who went on to become a noted film star.
Marilyn Monroe as an aspiring starlet. Her last scene is being laughed out of her theatre audition and told to try televisionnote In the Golden Age of Hollywood, turning to television was considered a real step down for any actor. Not only did she become a superstar in real life, she never starred in any television productions.
I Am Not Spock: Due to Bette Davis being the lead and on the cover or poster, underneath the title - many first time viewers get a surprise when they learn that Davis's character is not called Eve.
Nightmare Fuel: The concept of someone slowly destabilizing your life and your nearest and dearest friends not believing you when you turn to them for help makes Margo's frequent angry outbursts very understandable.
Values Dissonance: Addison's scathing comments to Marilyn Monroe's character about how she has no chance in theatre and must resort to television. Back in that time, television was quite new and considered a real step down for any actor - rather than the easier way to get famous it would become (in terms of theatre acting anyway).
Unfortunate Implications: It has been argued that this movie has anti-homosexual and anti-feminist undertones. Addison and Eve are presented as villains, and, as noted, are often interpreted as homosexuals. Eve's focus on her career, in contrast to Karen's devotion to her husband, and Margo's eventual acceptance of Bill and of her fading career, is shown as devious, and she eventually succumbs to Addison's domination. The Other Wiki has full details.