"Professor, we've got to barricade the school, he's coming now!"
This quote is significant because it shows that there is going to be a climactic finale to this book, and this quote helps build the suspense and anticipation for it.
Simile: "... and tiny little ears like a hippo's." Luna Lovegood says this on page 511.
Onomatopoeia: "CRASH". This sound is describing the sound of coin and treasure falling on page 537.
Metaphor:"... he was Voldemort". This is when Harry compares himself to Lord Voldemort by use of a metaphor on page 548.