As far as contrast goes, I'm with janeyb here--the most noticable thing is not the contrasting details in most of the characters as much as how interchangable the main characters are. If there is contrast, it would be in the "exemplary" characters--the Duke and his intended, the fairy king and queen and (ok not exactly exemplary...) Pyramus and Thisbe. Titania and Oberon, as faerie royalty, might be expected to peacably woo, but their marriage is fraught with power struggles and strife. When they finally do make peace, it is not out of absolute free will, but as a result of Oberon's trickery. In contrast are the "happy" couple Hippolyta and Theseus who met in battle (Hippolyta was an Amazon Queen before Theseus conquered her people and "won" her). She may be engaged to him but, nonetheless, she remains a spoil of battle.
Finally, Pyramus and Thisbe are the staged representation of lovers who love truly for the time they have but end tragically. And, as the players are wont to point out, they are not real.
These exemplars of mature love, together, form a strange frame for the young lovers, for not one of the loves is free from violence--be it beginning, middle, or end.
Although Midsummer is clearly a comedy, the other loves leave the audience with a growing awareness of the unfairytale-like reality of love.
Contrast is one of the major techniques that Shakespeare uses to accentuate the characters of the play. The technique contributes to the surrealism and comedy of the play by accentuating each contrasting characteristic. An ugly person is never so ugly as in the presence as one who is beautiful.
The play is full of contrasts. Helena is tall, Hermia is short; Puck plays pranks, Bottom is the object of pranks. Titania is beautiful, Bottom is ugly. The fairies are delicate and graceful; the craftsmen are stout and clumsy. One of the most memorable scenes of the play is the beautiful Titania weaving flowers in the hair of the grotesque Bottom. Such extrordinary contrasts are present in every scene making contrast one, if not the central, motif of the play.
I think a better question would be one regarding the lack of contrasting characters in "A Midsummer Night's Dream." Although the older couples in A Midsummer Night's Dream have greater depth than the Athenian youths and their female mates, none of the characters in this play is truly three-dimensional. Think of Helena, Hermia, Lysander and Demetrius. They are so alike, even in the sounds of their names, that they are constantly being mixed together and apart to further the plot. Here is a small excerpt from the enotes on the characterization of the four Athenians:
"Most commentators, however, argue that this lack of individualization is central to the plot, that Shakespeare did this on purpose. The young Athenians may seem indistinguishable to the audience, but as objects of love to one another they are seen as sheer perfection. Arguably, it is the transformative power of love that makes four almost identical people seem so different and so wonderful in each other's eyes. On the other hand, Shakespeare may have painted the young lovers as he did in order to highlight the folly, capriciousness, and inconsistency of their love. "
The plot as well as the characters are overwhelmed by the beauty of Shakespeare's magical lyricism. The contrasting nature of the characters is visible only to those who love them.