I'll be really interested to see what goes in those guidelines.
What would I put in 'em?
1. Make it modular. Give it barely enough backstory to justify its own existence, don't try to shoehorn it into a pre-existing setting (or if you do, make the setting ties as loose as possible), and design it with the intention that any DM will be able to take it and drop it into her own game and-or setting with minimal revision.
2. Make it one single adventure. Too many times have I seen adventure modules written like a book - chapter 1, chapter 2, etc. - which is fine if the players are willing to go through the chapters in order and completely pointless if they don't. If you find you're writing chapters, write them as smaller individual adventures instead.
3. Give it variety. If it's playtested 10 times and each party approaches and-or defeats it differently you have a winnah. Have more than one entrance. Have multiple connections between levels. Have some foes that stay put and others that move around. And so on.
4a. Follow tradition. Include at least one standard D&D trope - a pit trap or two, an illusion, a dragon - something that makes it obviously D&D.
4b. Break tradition. Make sure your adventure includes at least one element you've never seen before - if you're an experienced player/DM and you haven't seen something, chances are most others haven't either. A new monster, a new spell, a new class even...but something new and - with luck - memorable.
5a. Make sure the maps agree with the written descriptions. This should be obvious. Too often it isn't.
5b. Make sure the map is clear about what is where, with a scale and compass. Again, this should be obvious...
6. Make sure the "boxed descriptions" make sense no matter which door the party enters through. Far too often the writers assume entry through door A, forcing the DM to frantically revise it on the fly when the party comes in through door C.
7. Put all the info for a given encounter/room/area together. It's just easier to run that way.
8a. When in doubt, write less. Keep descriptions short and stat blocks shorter.
8b. That said, try to cover some of the more obvious what-ifs. What if a party has flight? What if the party is all the same class? What if they let the demon loose rather than leave it alone? What if they try to open the gate to hell rather than close it? What if they melt the only key to the princess' cell in a fireball, or accidentally shatter the artifact they were sent to find*? Etc.
* - I have played in a party that did this - man, did that DM have to improvise!
How's that for a start?
Lanefan