Game playability is a vital concept which most current game designers understand and practice. Unfortunately, Most focus on system playability - making the systems easy to understand, appropriate for the game and setting, and flexible enough to cover anticipated play. This is a laudable goal, and one not always a goal to the previous generation of designer. A different goal, and one I see met far less frequently, is Setting Playability.
Settings designed without keeping the goal of Setting Playability in mind can fall into what I call End Traps. End Traps are where the concepts behind the setting are pushed to a logical conclusion and become metastable. The settings cannot be affected by the PCs in any meaningful way. This frustrates groups who may like the setting otherwise. Settings need to be dynamic - stable enough to last, but not so stable as to be eternally unchanging.
Another potential trap is Setting Paralysis, where the setting is so different in every way from anything the group has seen before that GMs and players are mentally stunned - they have no handle to grasp, nothing familiar to ease their way into the setting. It's always good to have some familiar bits as well as some far out stuff in a setting. That way those unfamiliar with the basic tropes can have an opening into the setting.
Of course a huge problem with established setting is Setting Buy-in. This is where there is so much to learn about in order to get going that it all becomes more like work than play. This one is difficult to get around, because older setting just accumulate cruft. Even settings which are new to games may have been around a long while as TV shows or books. The difficult trick is to find the right balance between Too Much Information for newbies and Not Enough Information for existing fans.
Just some thoughts