The Forge is a discussion-driven website that focuses on analysing games and actual play, and on independent game creation (ie, author-owned and home-made games). It is run and maintained by Clinton Nixon and Ron Edwards.
A few discussion forums have "sticky" threads for policy and links to highlight particular threads; the latter are well worth reading: eg What's the best way to present a mechanic for discussion?, The Infamous Five and the Forge News Thread, which is worth watching (marking to receive updates by email).
John Kim maintains a Forge subsection on his site, including thread highlights, at http://www.darkshire.net/~jhkim/rpg/theory/theforge/
Before diving into discussion you may want to read the articles that explain some of the jargon used on the Forge (these build on each other, so it's recommended that you read 'em in order). If you don't care for such stuff the best forums to start in (depending on what you're looking for) are Indie Game Design and Actual Play.
System Does Matter
http://www.indie-rpgs.com/articles/11/
GNS and Other Matters Of Roleplaying Theory
(definitions, multiple click-through pages)
http://www.indie-rpgs.com/articles/1/
Simulationism: The Right To Dream
http://www.indie-rpgs.com/articles/15/
Mongrel
example Sim game, PDF
http://www.indie-rpgs.com/files/mongrel.pdf
Gamism: Step On Up
http://www.indie-rpgs.com/articles/21/
Black Fire
example Gam game
http://www.indie-rpgs.com/articles/22/
Narrativism: Story Now
http://www.indie-rpgs.com/articles/21/
Zero at the Bone
example Nar game
http://www.indie-rpgs.com/_articles/zero_at_the_bone.pdf
More articles
http://www.indierpgs.com/articles/
Reviews
http://www.indie-rpgs.com/reviews/