someone who doesn't grasp that others don't know what they know I once heard of a disorder that described someone that didn't readily understand that others don't know what they know.  So, the person would often start conversations with vague comments about esoteric concepts then become aggravated when other's weren't familiar.  Does this have a name?
The skill of understanding that other people have different knowledge, beliefs, etc, falls under the concept of theory of mind (note that the name is possibly a bit misleading...it's not referring to some scientific theory or to philosophy of mind but rather to this ability to understand that others have different minds).

Difficulties with theory of mind are a common aspect of autism, but can arise from other causes as well including schizophrenia. I don't know of a specific term to describe precisely the symptom you are describing, however, and there may not be such a term.