one of the main things I look at is pacing....i get a rough idea of what I want (sure you may think "i cant think of anything original just the same old schlock") but its the details that make it different... The details help with pacing.
You go there: what about the obstacles of getting there, anyone else going there, do we even know where "There" is, Is it even possible to get "There" witht he means we have.
I Put Extra pacers here just in case...throw in a couple side things that may or may not go anywhere...I put a burial mound in one adventure that had a rats nest in it and a skeleton...thats all, the players spent half an hour doing things with it...they dug it up rats attacked, they killed the rats, they find a single skeleton with a leather purse around his neck....they argue over who is touching it or not touching it, they open it, and find 2 gold coins...they argue over if they shoudl take them or not and hell one guy kept wondering what magical powers he got, and it didnt help that the players kept egging the guy on saying it was a 6 demon bag....
Kill that guy: can the pc's kill him, hell can they even find him for that matter, has someone else already killed him, does it take special means to kill him, should he be killed, is he the right guy.
More extras, A doppleganger is something fun to throw in, because the players have to decide what the hell is going on if they were sent to kill a specific person....
Retrive this thing: can they, is is distroyed, the size of a elephant, locked onto something else, alive and moving, is there a certin way it has to be removed...
even the most basic ideas can be changed around and added too, to make it something totally new...