BG failed, not because of budget (which was probably 10 times that of Babylon 5 (B5)), or acting, cast changes, director problems, or writing (which is some of the best I've ever seen), but because they didn't have a plan. BG and Lost claimed they had a plan, but didn't. On the other hand, the creator of B5 shorted out all of the episodes for 5 years, so he knew where things were going from day 1. B5 is the only show that I know of that has done that, and it is (in my opinion) the only way to do long-running shows like these. Why does this make such a difference?
- Inconsistencies. When you have a plan, you don't have nearly as many inconsistencies. You're not trying to hack together a concluding episode, trying to make all of the lose ends tie together in some haphazard way, because you knew how they tied together at the beginning. Star Trek Next Generation solved this problem, mostly, by not having any long-running plot lines. Each episode ends with the characters in exactly the same condition as they started. BG had some good ideas, but its resolution seemed like a hack.
- Foreshadowing. When you have a plan, you can have foreshadowing. Not just vague, unexplained claims, but real references to future events. In the first episode of B5, a character refers to a prophetic dream they have about their death 20 years from then. At the time the viewer thinks they understand the dream, or think that it is a toss-off comment, but late 3rd season we see some of the details, and late 5th (and last) season we see the lead-up to those events. The flashback method is the poor-man's (or poor planners) way of doing this, basically saying "we need these events to have happened in the past, to make sense of what we want to show now, but we didn't think of it before so we'll show it now". BG used this a lot.
- Larger Story Arc. Even at its weakest points, B5 benefited from its plan. A weak single episode was lifted up if it contributed to the larger story arc. In this way, weak episodes were improved because there was a plan, and the entire series seemed more consistent.
- Rewatchability. From a marketing point of view, this seems to me to be a no-brainer, although it is almost never done. Once B5 finished, I wanted to go back and watch it again to pick up on all of those things that were planned, but I missed. How much foreshadowing did I miss? How many little details in the background were there that became important, but I didn't realize? How many decisions of the characters contributed to their final roles? How many seemingly throw-away lines were really important? Now that I've finished watching BG, I don't have any urge to watch it ever again. There's nothing new to see. Finding out in season 3 that Tigh is a Cylon doesn't modify your perspective on any of his actions in season 1. None of the fates of the characters can be seen in the early parts of the series. The so-called prophecies are each stated and resolved in a couple episodes, or so vague as to be meaningless.
I've heard that Lost suffers from this same problem: no plan leads to inconsistent storylines, convenient flashbacks, and an unsatisfying conclusion with loose ends.
Now, B5 wasn't perfect. It could have used more money, better actors, better dialog. It's writing can be corny at times, and there was a studio snafu that condensed some of the plot in Season 4, and made Season 5 a bit thinner than one would like. However, the universe is entirely original, and the 5-year plan was just amazing and makes the series hold up well over time.
I now won't watch much of anything without a plan...it's just not worth the ride.