Michael O'Hare, who played Commander Sinclair, suffered from severe schizophrenia. It became steadily worse during the first season, causing delays and other complications. Straczynski was willing to work around it, as best they could, but O'Hare chose to step aside.

Sinclair had been intended to carry a lot of the story arc. He was supposed to become "The One" who would unite the younger races to fight against the Shadows. There were various things in the first season that built toward that.

Without Sinclair, most of that had to be retconned to fit Sheridan instead. It was carried off surprisingly well, but suddenly having two messianic characters complicated the storyline in some odd ways. It felt a little awkward at some points.

I think the bigger problem was the network jerking them around. The Great War and its aftermath were supposed to be the series finale, but they ended up rushing to finish it a season earlier than planned, thinking they were being cancelled. When they were actually renewed, they had to come up with something to do for season five. It didn't feel like the same show, to me.

Overall, the series was still a triumph. It told an original story about a large ensemble of fascinating characters in a way that had not really been done before. I just think it could have been greater if it hadn't had to make so many adjustments along the way.