Full Idea
If it is central to 'proposition' that there be quasi-syntactic structure, so that there are subject-predicate, or negative, or conjunctive, or quantified propositions, then sets of possible worlds will not do.
Gist of Idea
Propositions can't have syntactic structure if they are just sets of worlds
Source
David Lewis (On the Plurality of Worlds [1986], 1.5)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.57
A Reaction
He proposes 'more complicated set-theoretic constructions out of possibilia' instead. I am very much committed to propositions having quasi-syntactic structure.