What was Mill talking about when he wrote about the quality of pleasures?
Mill never discussed the quality of pleasures, he though that only the quantity mattered.
He was talking about how much you enjoy a particular pleasure---Mill said that the more pleasure that results from an action, the better.
He was referring to types of pleasure: specifically intellectual versus physical pleasure.
Talking about the quality of pleasure wasn't part of mills theory, it was part of Aristotle's.