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.
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.
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.
Signature Scene: The Expectations vs. Reality scene. In a slight degree, the park scene in day 488.
Tear Jerker: The "Expectations vs Reality" Party scene is a real kicker though, esp. for anyone who has had a similar experience.
As is the last scene at Tom's spot in the park.
Tom's heartbroken speech about love during the board meeting.
The 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.