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.