Firstly, you can talk about the role of coincidence as a stylistic element in this novel. In particular, coincidence is shown to be key way in which ruin is brought to characters. Consider the way in which the furmity woman returns to Casterbridge at just the right time to cause the maximum harm to Henchard. In the same way, the weather just happens to turn at the precise moment when it is able to ruin Henchard completely.
Secondly, you might wish to consider the role of Gothic description in this novel and how it adds a ghastly, supernatural note of horror and suspense. The best example of this is the role of the old Roman amphitheatre where Henchard and Susan meet. Before this meeting, the narrator relates to us a story that gives us the setting of this place and how it was used throughout the centuries. The clear reference of supernatural horror and violence adds Gothic touches to this novel.
Lastly, it is important to comment on a key aspect of Hardy's style in many of his novels, which is the creation of his setting. Wessex is a county that doesn't actually exist, but is one that is used by Hardy as the setting for nearly all of his works. This unity of focus gives a real sense of the richness of the setting and how Hardy wrote about his home location that he knew so much about. The added sense of reality and how the description builds up this imagined location is definitely a key component of his style.