In so many words, the film was an insult to the book. They essentially changed the entire plot and expected no one to notice.
SEMI-SPOILER ALERT
It was disappointing to see all the impactful and touching moments between the characters completely left out of the film. The game of assassin, which essentially sets the plot in motion is completely missing which in turn dampens the bond between John Ambrose McClaren and the wonderful, Lara Jean Song Covey.
Subtle details that made the book heartwarming like the necklace, snowglobe etc.. were fiddled with, which would not have happened if they stayed true to the source material.
Lastly, it hurt to see Netflix replace TATBILB's original director Susan Johnson with her former cinematographer Michael Fimognari. A man who changed the whole feel of the series and quite frankly doesn't know anything about being a woman. Whatever the reason for Johnson not directing the film, they should have brought in a female for a more authentic look at a teenage GIRL protagonist.
IT'S TOO BAD HE DIRECTED THE FINAL FILM OF THE SERIES AS WELL!!!!