"Harry would have stunned the Acromantula, but he didn't have enough mana."
"I knew I should have prepared Stupefy this morning," he muttered.

This is my favourite GoF chapter, too, and I think you've just managed to emphasise why. It is the writing, as well as Peter's Awesomeness that endears me to it. Though that said, the rereads make me realise exactly how many adjectives JKR uses.