James Joyce: The Difference Between Portrait and Ulysses and Finnegans Wake

Well, one difference, as explained by A. Walton Litz in the pretty good book The Art of James Joyce:
A process of selectivity harmonizes with his early notion of the ‘epiphany’, which assumes that it is possible to reveal a whole area of experience through a single gesture or phrase. In shaping the Portrait Joyce sought continually to create ‘epiphanies’, and to define Stephen’s attitudes by a stringent process of exclusion; later in his career he attempted to define by a process of inclusion. The earlier method implies that there is a significance, a ‘quidditas’, residing in each thing, and that the task of the artist is to discover this significance by a process of distillation. In the later method it is the artist who creates the significance through language. Thus in the Portrait a single gesture may reveal a character’s essential nature; but in Finnegans Wake Humphrey Chimpden Earwicker’s nature is established by multiple relationships with all the fallen heroes of history and legend.

Litz doesn’t force this dichotomy too much, which is good, but there is something to it. The single-moment emphasis in the early work gets contextualized and put into perspective in Ulysses before evaporating completely in Finnegans Wake, so that every moment becomes co-extant with every other moment. (And this very process, this consubstantiality, is a major theme in Ulysses.)