Dungeon Crawl
Most of my attention will be given to a few key rooms; these will be major encounters that relate to the purpose of the crawl in the first place. Descriptions, monsters, traps, with the first being quite important - dungeon dressing and the little things that make exploring interesting rather than one combat after another.
Agreed. Although note that "dungeon crawl" can actually mean "travelling through a designated area that is full of baddies and likely not too many friendlies or safe resting spots" whether it's a dungeon, an abandoned monastery, the path through the mountains, or the goblin forest.
All styles vary, and some styles work for others and some not, so I will tell you what I do and please steal what you like:
I don't make detailed maps, other than perhaps of key areas.
What I do is (and these are not in order - I usually am writing in a notebook and jump between pages scribbling when I get ideas)
1. Brainstorm and then kind of finalize the basic storyline/motivations - don't forget to think of the 'third option'. The 'third option' is the one you think of after you start with the most obvious two alternatives and discard them.
2. Think of what sort of creatures (and generally how many - a lot, a little, a few) will be there based on the storyline, and usually try to think of a couple interesting-but-fitting things - like having whoever was there before the current guys trying to fight their way back in.
2.5. Jot down who might be where in the 'dungeon' (or building, or wild) in general. For example: maybe the hobgoblins and goblins are the patrols and so likely to be encountered out and about.
3. Write down as many clues to the story and 'flavor' things you can think of, to be inserted during play at opportune times (you won't use them all). For example: The LBEG has a wand of fireballs? They might run across a tunnel that has a couple crispy bodies on the floor. It could also include notes, symbols, carvings or drawings on the walls, a hidden niche in the second in command's quarters, etc. The key here is to make no one clue that is most important - these are the little things that build up the ambience of the area or give clues toward the storyline, without the players necessarily knowing which is which.
4. Write down as many things that you can think of that seem 'interesting' to play through. Several different kinds of traps, stuck doors, things to break up the fighting like bookshelves with tomes to explore, prisoners or someone that's stuck holed up in a secret room because they are afraid to make a break for it, and so forth. This also includes potential details about the sorts of rooms/areas you'd expect to see (perhaps on a separate list) - storerooms with casks of ale, or a open area in the woods that's an obvious recent camp, etc. These are also to be inserted into play at opportune times.
While you're writing and as soon as you think you're done, read through everything you've written and add/edit whatever comes to mind.
Then, just before (and during) play, you can glance through the lists for inspiration while they are running through the dungeon. I've found that I only occasionally refer to the lists while we're actually playing - simply generating them gets me in the mind of creating things on the fly.