Vyvyan Basterd
Adventurer
You don't need to be bad player or person to read "default" material in "core" rulebooks and assume it is true for the game your signing up for. Many folks just assume that anything presented as default is just a part of D&D. Certainly 4e weaves its cosmology very tightly with its classes and motivations. You might choose any number of classes (or features) presuming that you will get to involve yourself in one its cosmological struggles.
Firstly, we all know what happens when one assumes...
Especially in a game known for individual table customization.
I was referring to the "fluff-lawyer" context. A "lawyer" in this context would assert that the books say what they say and demand adherence.
A good player could come in with assumptions, but will gladly go with what his chosen DM has crafted, speak with the DM about the vision he had to come to a compromise or excuse himself to search for a game that suits his tastes.