Bartle's two main flogging points:
* Players in virtual worlds are going through a process of identity development.
* Virtual world designers need to understand why decisions were made a certain way in other virtual worlds, so that they don't blindly copy something that doesn't fit (and remain open to alternatives.) Design choices, even unconscious ones, affect what values the world promotes and how players will interact with it.
I'm a bit skeptical of the identity-development angle. I can see that the processes he describes take place (both evolution of play style and the development of personas.) I felt his section on the hero's journey was a bit overblown though. I'm not sure what he describes is as universal as he believes it is. On the other hand, it is true that both an immersive environment and the presence of other players makes for a combination with qualities distinct from message boards or multiplayer strategy games or single-player RPG.
As a guide to the space of design choices (and an overview of the consequences flowing therefrom) the book seems to do a good job. I can map post-publication virtual worlds (like Puzzle Pirates) onto many of the axes he describes. I did feel that a little more practical angle would have been good. OK, so virtual physics is hard. Do you leave all the corner cases up to the development team? Or can you model-check a description before they get started? (Bartle does mention some prototyping steps that the design team can take, but at a very high level.)