Some fans have theorized that the final conversation between Tom and Summer in the park didn't actually happen - with the audio commentary suggesting that it could just be all in Tom's head. But the director and screenwriters disagree on this.
The narrative seems to view Tom as an immature Dogged Nice Guy who has an Entitled to Have You attitude. Tom admittedly does have an unhealthy view of Summer. The events in the movie is only shown from Tom's perspective and consequently we never get to know Summer as a person. Summer makes it clear that she's not looking for a serious relationship with him but he still throws a tantrum when his relationship with her inevitability falls apart. He never describes what parts of her personality he likes either, merely describing her physical appearance. At the same time, while Tom does overestimate some of Summer's actions, from his perspective she seems to give him little bits of false hope. It seems that Summer could possibly be aware of how much of a destructive presence she is in Tom's life and breaks up with him partly for this reason. It would explain her comparison of their relationship to that of Sid Vicious and Nancy Spungen, comparing herself to Sid.
Among fans of the movie, there is a clear divide between those who empathize with Summer and think Tom is an immature, selfish, delusional Yandere who unfairly projects the person he wants Summer to be onto her, versus those who empathize with Tom and think Summer is a selfish bitch. Interestingly, younger viewers tend to fall into the latter category, and many people say they empathized with Tom when they were teenagers and switched sides upon rewatching as an adult. Tom's actor Joseph Gordon-Levitt has made it pretty clear he's in the pro-Summer camp.
With regards to Summer's character, did she enter into the relationship thinking it was only a casual fling and hoping that Tom saw it that way (and the break-up brought on by the realization that he didn't)? Or on some level did she feel sorry for Tom and kept things going on longer to spare his feelings? For that matter, dancing with him at the wedding after they'd broken up - is it because she assumed they were Better as Friends and he wouldn't interpret anything romantic from it, or did she not mention her fiance again to spare his feelings?
Better on DVD: The brilliant use of Anachronic Order can be more easily appreciated if you can rewind and rewatch scenes and transitions.
Big-Lipped Alligator Moment: The spontaneous dance sequence (the morning after Tom and Summer have sex for the first time), which is simultaneously a glorious movie musical dance scene and a hilarious parody of movie musical dance scenes.
Common Knowledge: Some viewers - infamously Anita Sarkeesian of Feminist Frequency - think this is a straight Manic Pixie Dream Girl movie. This ignores that it's a Foregone Conclusion Tom and Summer won't end up together, and the Aesop is not to expect another person to improve your life based on superficial traits.
Ensemble Dark Horse: Tom's little sister Rachel, played by Chloë Moretz, possibly the only person in the movie to realize that the Manic Pixie Dream Girl trope is not reality. Though ironically, as a child giving relationship advice to an adult, she herself is a person who doesn't exist in reality.
Fanon: Due to the Fourth-Date Marriage some people have assumed that Summer met her husband while still seeing Tom - and thus broke up with him for this reason. Word of God has denied this. Doing the math confirms she got married six months after the break-up, which is fast but still perfectly plausible.
Rachel's Wise Beyond Her Years nature is even funnier with the screenwriter confirming that she's based on his real little sister - and the advice she gives is Truth in Television.
Rachel saying she knows "more than you" to Tom about PMS is darkly amusing when you see Chloe Moretz in Carrie (2013) - where she gets her period late at the age of seventeen and doesn't know what's happening.
Hollywood Homely: It's said earlier on how Summer is out of Tom's league, and a Jerkass at a bar reacts with shock at them dating. This is ignoring the fact that Tom is played by Joseph Gordon Levitt. He's dressed down in unflattering clothes and a slightly nerdy hairstyle but it still seems odd that Tom is supposed to be considered unattractive.
Jerkass Woobie: Tom's misunderstanding of Summer and their relationship is his fault; but it's never out of maliciousness, and he does genuinely care for her, and it's hard not to feel sorry for the guy. That being said, his Entitled to Have You attitude is shown as very damaging and a little dickish on his part.
"Expectation vs. Reality", which became a popular shorthand for things not turning out like one imagined that they would.
"I love The Smiths!" and the overall Elevator scene, usually as a commentary on shared music tastes and hipsterdom. People remain divided on whether the scene is charming or rage-inducing.
Misaimed Fandom: Though he is a sympathetic character, Tom is being very childish in his expectations and is making some big mistakes, even if he's not doing it intentionally. Some people didn't pick up on this Aesop resulting in this trope.
Narm: Vance describes his wife as "the light that guides me home." He's not remotely ashamed that he lifted this directly from one of his company's greeting cards.
One-Scene Wonder: Tom's blind date, who has enough of him bitching about Summer. Calling Tom out is definitely worthy of a Moment of Awesome.
Signature Scene: The Expectations vs. Reality scene. In a slight degree, the park scene in day 488.
Spiritual Successor: It has a lot in common with another recent movie that deconstructs the romantic comedy genre, 2008's Definitely, Maybe, including but not limited to a flashback-centric narrative structure, an ambiguous, less-than-happy ending, and a female lead named "Summer". They both also feature girls who are Wise Beyond Their Years that the main character is telling the story to (the protagonist's sister in this case and his daughter in Definitely Maybe.)
A romantic comedy that's about love and not a love story where the protagonist doesn't live happily ever after with the girl from the beginning with Anachronic Order? Sounds a bit like Annie Hall for Generation Y.
for things not turning out like one imagined that they would.