When I purchase a prewritten adventure, I understand that it will be written in a manner that will cover most contingencies that could come up. Cover as many bases as possible type of thing. I expect details even if most of them will never come into play.
When I flesh out an adventure for my own use, I just need "memory signposts". Just enough written down to jog my memory when the times comes. Heck, I find that if it's a common enough creature in the encounter, I just write the name and move on - in theory, i should be able to recall the AC, HD and damage of the average orc or even troll - and if I am off in one way or another, it just helps in keeping the players on their toes.
I am partial to using +James Raggi 's Random Esoteric Creature Generator for Classic Fantasy Role-Playing Games and their Modern Simulacra for the BBEG and / or it's minions on occasions. Again, a great tool to keep the players off balance, but I would use it lightly. If your otherwise "meeting typical D&D expectations" styled fantasy world has a bizarre creature behind every door, the bizarre will become normal and lose it's effect.
In any case, how detailed are the adventure that you write for your own use?
Part 2 - Do you design unique creatures / encounters to keep your players guessing, or do you stick to the book?