Just some thoughts....
Make the submissions unlimited. Encourage people to shoot, learn, reshoot, improve. People should feel free to post images from day 1.
Encourage image submissions to be from the current month. I for one do not like seeing recurring images from people's archives. Not much of a "project" if all people are doing are going through their files. I've done it before too.
Here's where I depart from everyone - get rid of the featured photo for this forum and DON'T do a poll. Polls tend to become popularity contests and having any flavor of a "contest" will keep new members (especially those new to photography in general) intimidated to participate. I imagine if I just learned how take and upload pictures, I wouldn't want to share where there was a monthly poll/featured photo with a lot of long-time members giving each other at-a-boys.