I've been a member in these forums for over two years. I certainly haven't been the most active participant (I tend to dip in and out of game design for fresh air and added perspective for when I come back), and I don't have any published games under my belt, but I've learned a ton since I joined. This community is great!
I love seeing when new people join BGDF. I get a little nostalgic for the time when all of this was new to me. What I'd like to do is have this topic be a place where we post tough lessons learned, tips, processes, etc., that we've developed over the years to help our new members out.
I'll go first!
1) Less is more. In the past, if I had a problem with how my game was playing, my go-to solution would be to add something. A dice roll! A new deck! More abilities! Until eventually the game turned into a hulking monster of choices, systems, and mechanics that were brutally complicated and unfun. Find the essence of your design, then build around it.
2) Question everything. I've started with themes and concepts that I liked and would stick to it even if the mechanics I was using would've been better fit to another theme. Don't let your vision of your game prevent you from creating something potentially better. Because a lot of the time, your game will barely resemble your initial concept. And that's okay!
3) Playtest as soon as you can. I've had designs that I would playtest because I thought they were incomplete. I'd spend so much time making it perfect and when I did playtest it, it was a disaster.