One of the things that makes this story, in some folks opinion, more than a simple love story with a happily ever after ending is the absolute importance of class over nearly everything else in 19th century England.  So much of Oak's efforts are to move himself into a position socially where his love for Bathsheba could actually be accepted by the wider society.

A second theme that elevates it above a simple (nothing wrong with them mind you) love story is the constant cliff-hangers.  Disaster lurks around every corner but Hardy insinuates that sometimes those disasters can also be avoided if proper care and effort are applied to the problem before it is too late.