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.
YMMV: World's Greatest Dad
Awesome Music: Lots of the soundtrack, which contains both recent, indie songs and some more classic songs (Peggy Lee). Also, Bruce Hornsby.
Harsher in Hindsight: Lance Clayton states that suicide is a permanent solution to a temporary problem, and that it should be avoided. Robin Williams committed suicide in 2014 - and his death was attributed to asphyxiation, which is how Kyle dies in the movie.
Heartwarming Moments: The very, very end where Lance, his neighbor, and Andrew (Kyle's only friend, who knew Lance faked the note and the journal all along) watch zombie movies together as fellow outcasts.
He Really Can Act: Robin Williams had been doing serious movies for decades and won an Oscar over 10 years before this film, but it was still a far cry from the lowest-common-denominator stuff he'd been doing over the past few years.
Misaimed Fandom: There are posts on this film's IMDB board about how "awesome" Kyle is.
Kyle's death, at least for Lance. The audience only cries because they're sad for Lance, not because they miss Kyle.
The pool scene at the end might make you cry tears of joy since it shows Lance is finally free. It's also somewhat sad because of the Reality Subtext: It's the last time we ever saw Robin's chest without the scar from his heart surgery.