What exactly shared means?

[via Digitalmars-d-learn]

On Saturday, 3 January 2015 at 00:12:35 UTC, Jonathan M Davis via 
Digitalmars-d-learn wrote:
> In D, if a type is not marked as shared, then it is by 
> definition
> thread-local, and the compiler is free to assume that it's 
> thread-local.

I find this to be rather vague. If the compiler exploit this to 
the maximum wouldn't that lead to lots of bugs?