The big difference between the 1989 Plan and this one is that from the beginning this Plan is intended to incorporate the fact of some 40 national space programs (Ecuador and Kenya both have nascent space programs), and that the path to space is no longer a single plan but a herd - for lack of a better word - of different entities with different goals, motivations and theories, but some common goals. Many entities want a lunar outpost for various kinds of research, including research on requirements for a future permanent habitation. There is strong disagreement on almost every aspect of that project. Nobody can say for certain that the first outpost will done by one government, international group of governments, commercial entities, or a combination thereof.
But some things are obviously true. The project will require various items, including a reliable contained space with air and other necessities of life, protection from cosmic rays, a way to produce food and recycle wastes as soon as possible, communications, various tools and materials for construction, maintenance, research, etc., probably some robotics to do the initial construction before we send people - and of course, a launch system to get the packages to the location. I tend to include the economic, financial, and political requirements as well. All of these pieces can be defined with projected costs, development times, and other aspects including their second-order requirements (what technical capability must be ready to put robots on the Moon?). And then we can connect the pieces up in various ways as a precedence graph - somewhat simpler than what Google Maps does to figure out your best route - to determine one or more 'best' routes based on criteria of cost, time, difficulty, probability of failure, etc.
So that's the new Plan - a comprehensive online tool for generating precedence graphs based on the various relations between elements. If the Raptor methane engine is required for the Falcon Heavy, and the schedule for the Raptor gets pushed, then so also does Falcon Heavy, and thus all the planned flight schedules, and thus all of the business or government activities that depend on them. The 2015 ISP poster is just a snapshot, limited by space, time, resources, and a lack of information. In today's world of space development, there are many, many different significant projects and so many different 'critical paths' - but the common goal is to get us, and Earth Life, into space and off the planet.