Some of the items are already implemented and some are planned.
Let me address each one here:
How do I see all Projects that don’t have a Next Action assigned?
That is not yet implemented.
How do I label something as “Waiting For”?
Not implemented. For now can be defined with tags.
How do I give something various tags/labels (other than the red/orange/yellow priority tags)?
You can add a tag by typing #tagname inside the node’s text. You can then search tagged items in the search-box. This functionality will be extended. We are also planning to add labels or similar functionality.
How do I define Next Actions at various sub-levels and sub-sub-levels of the hierarchy?
The first incomplete child of a Project is a Next Action. You can turn an action into a Project, then it becomes a sub-project. I will create a more detailed description in the Help soon.
How do I designate something as an Action?
Direct children of a node of type Project are Actions. Children of Action node are of type Node (not Action as it was in GTDNext).
How do I see a list of all Projects?
Not implemented yet