Actually, I have worked this one to both extremes.
1) Too much footwork.
The party spent days of play time asking questions, gathering supplies, and generally making a pain of themselves. Took them about three weeks until the expedition was ready.
Doing the research was good. But to say we need X to do Y whilst in front of people who had better networking skills than the wandering adventurers was just foolish.
By the time the heroes crossed the Dark Forest, Swam the Deep Swamp, and climbed the Steep Mountain; the Baron had promised the local Merchants Guild additional concessions if they would dispatch a mid-level group of Commando/Pirate/Ninja/Mages to return the Egg Cup of the Gods.
(I toyed with the idea of a shoot-out for control of the item and who got to return it, but settled for the party being a day late and a dollar short.)
2) Not enough, or Charge!
A basic rescue the princess from the dragon. Said dragon lives in an abandoned temple near a volcano surrounded by the Great Desert.
And they're off...
No one asked the color of the beast.
No one thought about scrolls/potions/rings of protections vs. various elements.
No one asked about back-up resources (theirs or the dragons).
No one thought about rations (each character has on their sheet 'one week- Iron rations'. Occasionally I 'tax' some of their loot to represent spoilage, munchies, etc.)
No one remembered the horse the princess was to ride. (Try spending six hours a day with two people in a saddle for one. Not happening.)
"Dragon slaying sword?" did not even cross their minds...
Swaggering into the lair resulted in 11 deaths out of 14 characters (it was a special request that I had two groups at once).
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Often it is really a question of considering what the players might do (and what their characters might do, often two different things) and planning the likely response.
I had mentioned elsewhere that there are many sources for good information that the players could access. Rather than re-hash that list again, I'll say this: any source you might use to find out more about something is fair game. By no means an exhaustive list:
Travel guides (people, books, agencies)
An embassy (yours or someone else s)
Merchants
Sages (Duh)
Mages
Other adventures (especially the retired ones. "Let me tell you about the time we...")
Street people (see everything- never noticed)
Street kids (Baker-street types)
Experts (Depends on what you need)
Your Guild or Union Hall (the expensive ring and secret handshake are good for something).
Soldiers (Been there, Killed that.).
and
'The party that almost made it but failed at the last moment' - If they're feeling nice that might tell you the villains secret weapon before it gets used on our heroes. Or you might have to buy them a round of drinks first. Or maybe just raise one of their fallen comrades.