Like any artist, as he improved his craft, he grew embarrassed about his early work, and felt it wasn't up to the quality of the later books. There are also some inconsistencies; most notably, Death is actively trying to claim Rincewind, when in later books He just facilitates the process of crossing over.
Notably, The Colour of Magic and The Light Fantastic both directly parody contemporary fantasy, and if you're familiar with the books he's making fun of, it's pretty obvious. Equal Rites is where he went more into the allegory, satire, and social commentary which people tend to associate with Discworld.