Race Conflict Metaphor
The issue pure-blood witches and wizards have with muggle-born witches and wizards can be a metaphor for the issues the Nazi had with Jewish people. In the Deathly Hallows similar measures are taken to control and eliminate muggle-borns and muggles. Muggle-borns are forced to register with the Ministry of Magic, much like Jews begin forced to identify themselves. Their property is confiscated, like the Jews, and many disappear. They are seen as inferior by some pure-bloods because of their non-magical lineage, similar to the way the Nazis viewed the Jews as inferior.
Patronus Metaphor
Patronuses are a true reflection of the witch/wizard. Harry's is a stag, because his father's patronus was the stag. Snape's patronus was a doe, because Lily's was a doe and he always loved her.
Dementor Metaphor
Dementors are a metaphor for depression. It is stated in the earlier that dementors literally suck the life out of their victims. It is also stated that the don't need walls or fences to keep the prisoners in Azkban contained, they are trapped in their own worst thoughts. This can be seen in the physical changes, especially around Hogwarts when the dementors arrive.
Simile
"But he thought of Ginny, and fear bubbled up like acid in his stomach." (p163)
Harry's fear for what would happen to Ginny, if she was taken by Death Eaters, upsets him to the point that it physically upsets his stomach.
