Mark Jaquith on dashes and hyphens in the editor:
So, three hyphens always gets you an em-dash. Two hyphens gets you an em-dash if spaced apart from surrounding words. Else, you get an en-dash.
I don’t mind the Visual Editor as much as some others seem to. Half of everything I write goes directly into the WordPress Visual Editor. Learning the editor’s behavior, like what Jaquith describes, makes the process quite a bit more pleasant.