Vincent Baker's blog "Anyway" has a ton of valuable information, including both theory and sales/publishing information. In any given post, there's a lot of smart discussion as well. A good theory/idea listing is right here. Vincent is a GREAT resource and you should ask questions! (I think he also has a good following on Google Plus, so there may be more discussions happening there, these days.)
The old Forge forums, no longer active, had quite a bit of useful information, though at this point, it's digging in archives. The Articles section (particularly on publishing) has some really good, and still valid ideas which have become a lot more accepted (electronic only game publishing, etc.) I've written a good summary of the big publishing ideas out of the Forge on my blog.
A fair number of folks who used to be on the Forge are now at Story-games.com, though due to the way the site is set up, it is a lot less focused and you will have to pick more carefully between "valuable and true information" vs. "everyone is really excited about this thing but doesn't actually know" kind of information.