Stephen there are some potential spoilers below so read at your own risk.
I ended up watching it as a result of one of these online quizzes posted here and quite enjoyed it.
The series has excellent continuity as the story line was planned out from the beginning.
Not sure I agree. They claim that now, but that might just be spin. While Season 2 onwards is definitely one massive story arc, Season 1 seems to be more stand alone in nature.
Without watching the first season over again here are a few examples of things that presage the future of later seasons.
Season 1, Episode 4: Infection - A living weapon from a planet destroyed by its own defenses after fighting off an alien attack. Hint of the last "Great War".
Season 1, Episode 6: Mind War - Catherine Sakai runs into a "First One" and is told of them by G'Kar.
Season 1, Episode 11: Survivors - first hint of Clarks conspiracy.
Season 1, Episode 13: Signs and Portents - first appearance of Morden, Delenns reaction to him is consistant with the "Great War" story arc. Also the bare beginning of Londos path is revealed. Londos path is consistent with his behaviour through this whole season, including his efforts to leave the path.
Season 1, Episode 16: Eyes - another connection to the "Clark conspiracy"
Season 1, Episode 18: A Voice in the Wilderness - The 5 century old installation on the planet waiting for the time to come.
Season 1, Episode 20: Babylon Squared - The first hints to what actually happened to Babylon 4
Season 1, Episode 22: Chrysalis - connects with Delenns ancestry which she later discovers and explains part of Valens origins.
All of these episodes leave links that are clearly picked up and woven back into the story line. Hard to do if it wasn't planned that way.