Most notably, Hardy uses an AABB "perfect rhyme" scheme.
A theme in many of Hardy's works, Victorian era social norms and constraints and Victorian society are themes in the poem.
Certainly, the theme of ruin runs throughout the poem. Melia has ruined herself through prostitution; she has no honor and no official place within respectable society, yet she has apparently risen from life as a farmworker to take on the apperance of a lady in that society.
Additionally, the speaker of the poem, the farm maid is "ruined" by her inability to assimilate into the Victorian culture as a function of her birth and breeding; she is common in both speech and appearance and covetous of Melia's apparently "ruined" position. There is irony in this.