NOTE: This may contain SPOILERS for Babylon 5, Battlestar Galactica and Lost.
I'm a big fan of episodic television with long arcs. I love pay-offs that are separated by months or even years from their initial set up, where you can go back and watch earlier episodes and see things you never noticed before.
Back when I was watching the X Files, it was always the so-called "myth arc" episodes I liked and never the "monster of the week". But it eventually became clear that even the myth arc episodes in the X Files were being made up as they went along. I felt duped.
So it was with great anticipation in 2000 that I started watching reruns of Babylon 5, a show famous for having been planned out in advance. Despite numerous flaws, this planning paid off multiple times. There were multiple OMFG moments where you realize how pieces of the puzzle from previous seasons fit perfectly together. In one classic multi-season arc there is a mysterious episode in season 1 involved time travel two years in the future. In season 3 you see the other half. Amazing stuff that would be very hard to pull off without the show being planned in advance.
Of course there are dangers with planning in advance. B5 suffered from story changes due to actors leaving.
This brings me to two of my favourite shows on TV at the moment: Battlestar Galactica and Lost.
Lost has supposedly been planned in advance and BSG largely made up as they went along.
If you'd asked me at the end of last year, I would have said the two shows were counter examples to the need for planning in advance. I didn't feel like Lost was giving me the payoffs and I felt like BSG was doing a great job without the long arcs. This year I've changed my mind. Lost is starting to get some pretty cool payoffs (although it could still screw things up) and BSG is breaking up under the stress of not being worked out in advance (although perhaps the finale will redeem it).
At least BSG had a definite end point planned for a while. There's nothing worse than a show that pretends to be teleological but is being written ad hoc with no real end in sight.
There are also opportunities available to the story teller when things aren't planned too much in advance. With BSG I didn't feel there were any truly amazing payoffs (again, the finale could prove me wrong) but there were certainly moments that were so out of left field and unpredictable it made for awesome story telling. It leads to a completely different kind of OMFG moment. Had BSG been planned in advance, those events might have been easier to see coming.
I guess that's the trade off. Plan in advance and you can get amazing payoffs but you run the risk of leaving too many clues so there are no real surprises. Write ad hoc and you can totally surprise the audience but also expose yourself to too many plot holes. BSG has one more shot to see if it can successfully close some of those holes.
Despite their flaws, I think Battlestar Galactica and Lost are both excellent examples of what works about each approach.
UPDATE: Here's a (spoiler filled) post on 12 Plotholes That Must Be Filled in the Battlestar Finale