My freelance workload is currently such that I prepared several adventure outlines at roughly the same time, then simultaneously built them into full adventures. This gave me a good look at my process, and specifically where a shortcut in my outline made much more work for myself in the adventure writing phase. So I though I'd share my list of "never do again" phrases from an outline (because I'm learning they make MUCH more work for me down the line). It's fine to punt on things like a specific number of orcs or exactly how many rooms the crashed spaceship has, but here are things that I regret punting until later.

"The PCs figure out that..." How do they figure it out? Does an NPC tell them? Is there a conveniently intercepted note? Does an interrogated foe tell them? Most importantly, have I built in the (very necessary) 3 ways for the PCs to find the solution to any mystery? The three paths should be spelled out. Better solution: Give more specifics about what the PCs use to reach a conclusion.

"...a bunch of..." This is bad wherever I put it. PCs fight a bunch of monsters? Well, which monsters? And in what order? And how do the monster encounters interact? Being more specific helps make the adventure writing MUCH easier! This is the same whatever the "bunch" is: the PCs find a bunch of treasure? How much treasure, and how much does it take of my total allotment? The PCs talk to a bunch of people. Who are these people, generally, and what do they impart, generally? Better solution: Don't defer a list of multiple items until later. Put the list in the outline.

"A villainous [noun]." Even as early as the outline phase--and, frankly, even before the outline phase, during the ideating and concepting phases, I need to know why my villains are doing bad things. They can't just be villainous in a vacuum, they have to be doing villainous things. The outline should spell out why villains are the bad guys, not just hand-wave that they're going to be bad and leave it to me to fill it out later. Better solution: A brief explanation of the villain's motives, so even the reader of an outline understand why the villain is an antagonist.

"The PCs solve a puzzle." This means I get to make up a puzzle. Yay, I like puzzles! But if I haven't decided how it fits into the story, I have to work all that out during adventure writing. That probably changes something in the outline I wish I would have done differently. Better solution: "The PCs are faced with [this kind of puzzle] with [this kind of outcome]. If they solve it, [result], and if they don't, then [other result]."

All these make my outline a little longer, but I've never been told, "your outline was too long." And they make my direction much clearer and my adventure writing easier. That's a win!