Full Idea
Mill's introduction of quality of pleasures into the hedonistic calculus is an unconscious departure from hedonism and a half-hearted admission that there are other qualities than pleasantness in virtue of which states of mind are good.
Gist of Idea
Mill's qualities of pleasure is an admission that there are other good states of mind than pleasure
Source
comment on John Stuart Mill (Utilitarianism [1861], Ch.2) by W. David Ross - The Right and the Good §VI
Book Reference
Ross,W.David: 'The Right and the Good' [OUP 1930], p.145
A Reaction
Mill argues that experienced people prefer some pleasures to others, but ducks the question of why they might prefer them. It can only be because they have some further desirable quality on top of the equal amount of pleasure.