My mind is totally blown at how much Lex, cpw, and co. have done with Forge since August. I'm totally at a loss at where I need to start with my plans for something I think will be really cool, but I've boiled it down to things I really need to know:
1. New @ tags, how they work, and how to make the best use of them
2. Packets: How best to use them, how to identify what needs syncing from server/client, how not to crash everything with a zillion packets
3. Rendering: Dynamic blocks, TESR's, etc
4. Making blocks talk to each other
5. Working with NBT for items and stuff
That's what I need to know, and if you deem it below you to answer me, just tell me where I can find it, that's all.