Perhaps you are having trouble selecting a system to use. There are many approaches that will help you narrow your choices down. You could pick a popular system in hopes that more GMs will use your adventure. To that end expect a future WoAdWriMo blog entry regarding the proper use of the d20 license and Open Gaming License.
Going in the opposite direction, you could use your WoAdWriMo adventure as an opportunity to shine a light on a system that isn't as popular as it could be. If you hope to evangelize for your favorite game it would be a good idea to write an introductory adventure for new PCs. Several very good games flounder in obscurity at least partly because some GMs don't know what to do with them. A good intro adventure helps overcome that obstacle.
Adventure writers that want to go systemless may find it helpful to introduce a little bit of system into their adventures. The Fudge role-playing game uses a seven step ladder of ordinary words to describe all traits. The ladder is as follows:
Along a similar vein one WoAdWriMo participant, known as VV_GM over at the WoAdWriMo forum, is working on what he calls the Universal Adventure Design Language. We'll keep you posted as details become available.
One final note about system choice. The good folks at Palladium Books seem to have a rather draconian and confusing policy towards online fan creations. While your WoAdWriMo support team loves many Palladium products, their internet policies and reputed litigiousness scares us just a bit. So while we heartily approve of anyone wishing to write adventures for any of Palladium's games, we will not be hosting such works at the WoAdWriMo section of Treasure Tables. But if you provide us with a URL to an alternative host, we will cheerfully link to your adventure.
[Many apologies to all readers for the lack of updates over the last week! My daughter came home with the chicken pox and I had been unable to finish this post.]