For discussion of (practical) implementation techniques, either for particular Bitcoin software, or that may be relevant to many implementations.

Bitcoin-related protocol proposals, or analysis or discussion thereof. For example, new P2P messages, new wallet techniques, methods for coinjoins or payment channels, etc.

Discussions about the game theory of Bitcoin, or Bitcoin’s potential impact on macroeconomics, or other Bitcoin economic topics. Maths is welcome, politics not so much.

More philosophical discussion about Bitcoin. Why is decentralisation good? Should you prefer to work with anonymous people? When is it okay to use a custodial wallet?