Harry's horrible guardians alternately ignore him and verbally abuse him. Vernon is the worst offender in this book. He spends his time threatening Harry and does things like tell everyone that Harry attends a school for juvenile delinquents. Now that's seriously juvenile. Otherwise, Vernon might want to watch out for his blood pressure – his Harry-induced rage doesn't seem healthy.
His wife, meanwhile, is her usual snobby self. Together, these two represent the absolute worst of middle-class Muggle society. They're judgmental and snobbish; they are driven by consumerism and materialism; they focus on status and appearances rather than things like family; they spoil their kid rotten; and they resent and fear anything different from themselves, namely Harry.
One interesting tidbit, though: Petunia doesn't appear to be a fan of Vernon's overbearing sister (Aunt Marge), who is essentially a female version of Vernon. This couple isn't always perfectly united.