What role do you have in mind for Discussion posts that you don't think should also hold for Main posts? (On your original conception of 'Discussion' and 'Main'.) Main-level posts like the Sequences also generally turn into active discussions, so I'm not clear on what is supposed to set Discussion apart.
The difference would be that it's only discussion, so you know what to expect.
Actually, a single Promotion axis plus tags would probably work better.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.