Over the six seasons, it seems like the show ended up suffering from "JJ/Good Times" syndrome. unlike Good Times, here Will was the main character from the beginning, but like Good Times, as the years went on he becomes nearly the exclusive focus, and the rest of the actors get the short shrift in storylines and character development.
Uncle Phil: in the early seasons, he was clearly the head of the household, and had an sense of personal and political history that informed his decisions. He experienced Jim Crow racism, he fought for a career as a lawyer during 60's and 70s civil rights struggles. He did not want his children to experience what he and Vivian went through to be educated and have careers of their choice. Whatever aspects of him that embraced the upper-crust lifestyle, he still maintained a sense of personal integrity and was not prone to being Will's foil for its own sake. The children clearly respected his authority, and vivian respected his opinion even when she disagreed.
In the later seasons, including after he became a judge, his career became deemphasized as well as his influence in the household. Frequently he existed only for being the butt of food/fat jokes, and his presence was barely felt in many episodes, amounting to one of Jazz's cameos.
Aunt Vivian: Early on, her career as a college professor was touched upon on a recurring basis (possibly UCLA, or another major school).. She, like Philip, came from a background of social activism in the 60s and 70s.. She frequently had to remind Will that his hip-hop-derived fetish for certain civil rights icons like Malcolm X didn't mean he really knew much about the man, and Will's fascination with the street life didn't mean he was morally superior or "more black" to those who live the middle-class life.
After being replaced by Daphne Reid, Vivian's career was almost totally deemphasized.. she became more of a background character.. And why couldn't she help get Will and Carlton into the college she worked for instead of ULA?
Carlton: Early on, Carlton had aspects of the preppy stereotype, but he had aspects of general maturity beneath the stuffed-shirt behavior. But in the later seasons (4 - 6), it seems like Carlton degenerated into a generic nagging 'nerd' scenario, reflexively sucking up to Philip, constantly having bad luck with women (in the earliest seasons he had dates/girlfriends), the hyper-obsession with elevator-music, the 'carlton dance', etc. One could almost say that the producers were trying to turn him into Steve Urkel without being super-obvious.
In an episode at the end of season 3 or 4, after reconnecting with his old neighborhood, Will abruptly decides that he misses Philadelphia, and tells Philip that he is staying. In the beginning of the next season, this is not dealt with dramatically at all; instead, a rather dumb 4th-wall sequence is created where Will is kidnapped by van (by NBC employees) and taken back to Bel Air. Will's antics/misadventures become increasingly juvenile in later seasons, despite having spent several years under the Banks' care..
Ashley: Initially portrayed as sweet and well-adjusted (and vulnerable to Will's suggestions), she ends up initially jealous of Nicky and being more of a typical bratty teenager by series end..
Hilary: Even after eventually getting the job as weather girl, then talk-show host, Hilary only barely evolved beyond her ditzy valley girl personality and worldview.
Vy/Will's Mom: At the end of season 5, Vy and Lisa's dad ended up in bed together, and married by the end of the episode. In season 6, this is just totally ignored.