Meadows' book is foundational to systems thinking as we understand it today; it's also part of the first generation of similar books trying to make systems thinking less abstract (see also Senge's Fifth Discipline in business for example). It is not, however, readily applicable to game design.

Her book has long been a traditional entry point for people to understand systems, and was a great inspiration for me in writing Advanced Game Design: A Systems Approach. My focus there was to write a game design text explicitly based on the idea that games are systems. An understanding of sources, stocks, and sinks, and of reinforcing and balancing loops, is just the beginning; there's a great deal more that can be understood and applied in designing games as systems.

Pesonally I would put "Thinking in Systems" in the same class as Alexander's A Pattern Language; Hofstatder's Godel, Escher, Bach; Luhmann's Introduction to Systems Theory; or even Wiener's Cybernetics as sources of deep systemic knowledge useful for but not directly applicable to game design.

I do hope however that more and more game designers begin looking at their games as systems, and that understanding this way of seeing and thinking helps create more effective designs.