I read this because I read Jo Walton's Or What You Will. It probably would have been better to read Twelfth Night first. Ha! 
I'm starting to recognize the themes common to Shakespeare's comedies: disguises, mistaken identities, humorous plots or pranks, frustrated lovers that get sorted out happily in the end. The clown steals the show; everyone else seems more foolish than the fool.
I have to agree with some of the other reviewers here that the "prank" against Malvolio goes far beyond a joke or anything he deserves, into the territory of outright cruelty. I have the impression that humor was crueler in previous centuries, and that this is one small way in which we have culturally evolved. A tiny bit.