Next Actions are distinguished as those items that are independent and available to be Done immediately. They may or may not be date/time related. When you look at your Calendar (first) and your Next Action List, you can go ahead and select an item and proceed because everything you are looking at is Doable.
Projects may be structured tree-like with steps (sub-Projects or Next Actions) that are parallel (actionable) or dependent (not yet actionable). Traditionally, when you plan a complex Project, you list out the steps and then arrange them in some logical order, noting dependancies. GtD Action Lists focus on the immediately actionable (independent) steps, but there is merit to retaining these notes - normally in a Project Support file, which may be an attached note to an electronic Projects list item.
The main distinction that I use is that the Action Lists are for Doing, while the Projects List is for planning. It's an important distinction for me because I can be tempted to revert to planning if all of my lists are visible - I need a limited Agenda to just get on with the work. I do not prioritize the Action List, but I do prioritize my Projects. In time planning, if I want to work through several steps of a Project in one sitting, I know that this preference will not be apparent from an unprioritized Action List, so I block out a Calendar appointment to work on the Project.