Season 1 was closer to tradition SciFi TV of the time (think of the Star Trek Next Gen.) Each episode is fully self contained and there is little in last weeks episode that you need to know to understand this weeks. There is a tiny bit of story moving from episode to episode (basically the Sheridan/Minbari/Battle of the Line bits and pieces), but mostly it is 22 episide of setup.

Seasons 2-4 move away from that construction to a fully searial story. its 90's TV so there still need to be a definite episode arc in most of the stories, so you still have pointless digressions like Garibaldi getting stuck in Grey 17, but the Shadow/Vorlon war takes over the bulk of the story telling followed by the Earther war.

Season 5 seems more like they did well on the previous seasons so while it was intended to end with season 4, they kept going but without a larger story to propel it.