One of the major themes across many of his artworks is time and our perception of it. In arguably his most famous piece, "The Persistence of Memory", Dali displays a multitude of melting clocks where it time is fading into oblivion. In this imagery, Dali could be making a statement on time in a dreamlike state fading or how memory can persist while time passes and melts away.
"Persistence of Memory" 
Often times, after returning from his "paranoid" state, Dali would want to immediately sketch out his hallucinations. Below is a sketch called "Fantastic Beach Scene" where there is a strange placement of arms within the sketch. It is cool to note his use of line thicknesses to accentuate important portions of each object and how they bring certain objects to the forefront and push some objects to the back.
"Fantastic Beach Scene" 
Another one of Dali's infatuations was the use of animals, specifically the elephant in his paintings. In this painting, "Swans Reflecting Elephants", the first thing that stands out are the swans basking in the lake. Upon further inspection, we see the reflection of the swans which like elephants. This hallucinatory imagery was a main staple of Dali and the surrealism movement where things can take on a second within the painting.
"Swans Reflecting Elephants" 
Impressionism as a genre has always been a favorite of mine as it is closest to my style of painting. However, surrealism and Dali provides an extra layer of thought and introspection that doesn't exist in impressionism which I really enjoy from an observer's standpoint versus a learning standpoint I might take when viewing impressionist paintings.