As much as I've enjoyed improvising content in game sessions, I've come to see some drawbacks to the practice. In short, while improvised content can be wildly fun and creative, it usually also tends toward a middle ground of risk and reward. This can sap a campaign of its sense of danger, challenge and achievement.
Consider the extreme case - a campaign where the DM makes up everything on the spot, under the eyes of the players. Assuming the DM is in possession of a full complement of social skills and mirror neurons, he or she will constantly, unconsciously be self-interrogating about the experience the players are having. Are they having fun? Do they see this as fair? Does the world make sense?
As sole authority, there is a strong pull toward the middle ground - to mitigate challenges, to clip rewards. The lurking spectre in the background is that of the juvenile, "mad god" style of DMing, where party-killing traps and mind-numbing treasures are handed out, "just because." Avoiding this spectre, you veer towards the safe and average. Giving out nasty surprises or extraordinary treasure would just feel wrong.
Another factor: the limitations of your mental co-processor when coming up with stuff in real time. Several times I have looked back on a combat that was improvised and seen how the party's enemies could have made a better go of it. The worms could have started tunneling when coming under arrowshot; the tribesmen could have been smarter about their ambush, doing hit-and-run rather than hit-and-fight. I don't discount the possibility of an unconscious sympathy for the players that makes it hard for monsters to do their worst, unless countered with a devious playbook, either written down or mulled over aforethought.
Committing plans to writing is one way to overcome the mediocrity of improvisation. Extensive thought, too, on the structure of a lair, the plans of villains and monsters, the likely action behind the scenes, often pays off with great results. With time alone, thinking, it's easier to convince yourself that the best-laid plans of that goblin horde necessarily involves putting the players in a near-deathtrap situation. Which in turn led to one of the best, tensest sessions of the campaign, where only ingenuity and tactical sense spelled the difference between a narrow victory and a TPK.
Another way to cope is to submit responsibility to the rule of the dice - also requiring written material, but this time a comprehensive table of encounters, traps, or treasures. It was with such a great sense of relief, several weeks ago, that I finally came up with my own treasure table, a task I'd been resisting partly just out of incredulity that there wasn't one out there I could use. So much more satisfying to leave treasure up to the whims of the dice than to create this little gold-star token world where "oh yes, you're 4th level now, you should be getting a nice little +1 sword..."
In a way, these admissions are uncomfortable. One main justification for using a simple and level-based rule system is to make improvisation easier, right? But maybe the best thing is to see it the same way as improvisation in music - great material for a cadenza or a solo, but ultimately dry and flaky without support the rest of the concerto or the rhythm section's steady groove.
(This just out: Entirely by coincidence, noisms has posted some very nicely complementary thoughts on the inadequacy of a preparation-free environment.)
Researcher in social science and appreciator of hexy wargames, role playing games, interactive fiction, board games, and CCGs. I've learned a lot doing design and rules editing work on the side for the Legend of the Five Rings CCG. But this blog is mostly about other things.