Can I add? I'm going to. Nuke this or move it if it's not fitting.
(Disclaimer: I've never finished a game to the point of wide publication. I have finished plenty of hacks and games used just for my playgroup and had a ton of fun!)
First, I want to direct people to a recent thread on Jonathan Walton's blog, since I greatly agree with it: http://thouandone.wordpress.com/2010/07/27/a-few-recent-thoughts-on-design/
But beyond that,Designers:
Try to play your game! Grab some friends or recruit your fans online (via Skype or group chat or something) and play the thing in whatever state it's in. I'm sure you'll learn something near-every time you actually play it that'll help advance your design.Fans
: You, too, if the designer's made enough info publicly available, try to run it. Try to keep notes and feedback about it to get back to the designer about how the rules and advice and so on were actually used (or not used) at the table.
So, just my 2c.