"Overemphasis and Understatement"
It seems that this chapter, more than any other in the entire series, plays up the Dursleys. They go beyond the point of being cruel, misguided people to an irrational family representing everything unholy in mankind. Take each of the seven deadly sins, as they're called, and you can find them in this chapter.
Pride -- their fear of their secret being revealed, their concern for the opinion of the community.
Envy -- Petunia is a notorious gossip, perhaps from her desire to equal her neighbors.
Anger -- the Potters. Enough said.
Greed -- Vernon throws himself into his work; is he really just money obsessed like most of the world?
Gluttony -- Vernon is huge. Eats doughnuts from the bakery for lunch.
Extravagance -- Perhaps hardest to support as they seem perfectly middle class, but we get the sense that they take great pride in and put much value on material things. This is more distinct in later books, however.
So here we have this family set up as the epitome of all things perverse, yet as the series continues, JKR proceeds to "humanize them" in a way. Their anger obtains a purpose, then fades slowly out, their prejudice becomes founded on rational facts and not theories, then eventually also fades as they obtain their final security. What are we supposed to get from this? Are people different than at first sight? That tends to be a common theme, but in the other direction. Do our perceptions change as we mature? Or is she just drawing us into the story?
Conversely, important themes and events that come up in this chapter are for the most part totally understated. Dumbledore, when making passing reference to Harry's magical scar, compares it to his own of the London metro system, saying "scars can come in handy..." (pg. 15). Also, this day is one of the most important in Wizarding history, certainly the most important in eleven years. Yet this feeling of celebration becomes extremely muted when seen through the eyes of a disinterested third party. To Vernon, these estatic Wizards are nothing more than weirdos (pg 3), and the even that they are celebration could not be important enough to affect him (pg. 8). This sentiment is echoed in the final description of Harry in the chapter:
"Harry Potter rolled over inside his blankets without waking up... and he slept on, not knowing that he was special, not knowing he was famous... He couldn't know that at this very moment, people meeting in secret and all over the country were holding up their glasses and saying in hushed voices: 'To Harry Potter -- the boy who lived!" (pg. 17)