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.
Hilarious in Hindsight: Just two years before the film came out, Brad Pitt and Cate Blanchett were slated to play the leads in The Fountain, but ultimately dropped out. In that film, they also would have played two star-crossed lovers dealing with the complications caused by unusual movement through time.
Hollywood Homely: Subverted as Benjamin refers to Elizabeth (Tilda Swinton) as "plain as paper". When we first see her she is indeed plainly dressed and nothing remarkable. However as her and Benjamin's relationship progresses we get to see her done to the nines on a few occasions.
Memetic Mutation: "Did I ever tell you that I was struck by lightning seven times?"
Benjamin is born in the shape of a baby but with all the physical features and ailments of an 84-year-old man.
How about when he's a kid with Alzheimers? Imagine if YOUR children began forgetting who they or you are? Alzheimers itself is Nightmare Fuel enough without the image of a young boy suffering from it.
In the short story, new-born Benjamin is described as "partly crammed" into the hospital crib— in other words, he was born a full-grown 70-year-old man. And somehow his mother survived the pregnancy!
The framing device: the arrival of Hurricane Katrina in New Orleans. As the movie ends, the sirens are going off throughout the city, floodwater begins pouring into the station - and the ancient clock once more starts running backwards...
Offscreen Moment of Awesome: Benjamin is all grown up in his mind, but has the body of a young man, and so he goes to explore the world and do all sorts of interesting things in exotic locales. But all we get is a short montage.
Sci-Fi Ghetto: For a (relatively) fantasy movie, it did great with the Oscars and critics alike.
They Copied It, So It Sucks: Many make this observation to this film in its similarities to Forrest Gump; written by the same screenwriter, about a guy's life as he deals with the love of his life, encountering many people and parenthood.
They Wasted a Perfectly Good Plot: Probably the most common criticism of the film is that the curious case of the title barely impacts the plot at all. Seriously, does no one in the world take any interest in this phenomenon?
Visual Effects of Awesome: Both the aging and anti-aging affects (a combination of makeup, CGI, and face/body mapping) were near-seamless, and won two of the film's Oscars.
Note that while Benjamin is the most extreme example, several other people in his life were also aged/anti-aged for the film, albeit in the normal direction.
What an Idiot: When Daisy crosses the street while dancing with her eyes shut, she is ran over by a car. While it is of common knowledge that the accident could have easily been avoided by the victim, the narrator introduces strangers marginally involved in the incident and carefully describes it as a coincidence instead of a potential Darwin Awards nomination.