1. The way vision and visual perception get tied up with memory. (Did they use the word "perspective" as a narrative word in the mid-19th century?)
2. DC's phantom imaginary sister—both Steerforth and Miss Betsy wish he had a sister, but instead he just has a dead, "girlish" mother.
3. Narrator keeps wanting to track what he knew—was aware of—at each point in the narrative. Likes to admit that he may be making things up; uses "I remember" as a kind of refrain. A kind of unreliability that, I think, may actually bring the reader in closer to the novel's world.
Also, it's fun. I like fun.