Pittsburgh is in southwestern Pennsylvania, on the western edge of what Pittsburghers (along with most other Americans) think of as the Northeast, but across the Allegheny Mountains from the East Coast. Partly because of the city's historical geographic isolation from eastern Pennsylvania and other large eastern cities, along with perceived differences between Pittsburghers and people from nearby Ohio, Maryland, and West Virginia, Pittsburghers tend to identify with the city rather than with the state or the region. This helps account for their belief that the local vernacular dialect is unique to Pittsburgh or to the immediate area.
The two factors that inform Pittsburgh as a place are the relative geographic isolation and the city's position at a crossroads of regional cultures. Consider this map of major dialect regions in the United States. Pittsburgh doesn't fall within any of the major dialect regions, showing characteristics of 4 or 5 neighboring areas: North, Midland, South, Mid-Atlantic, and New England. Pittsburghese is a creole of major regional dialects with linguistic artifacts from a number of American sub-cultures.
Perhaps most compelling is Dr. Johnstone's thesis that Pittsburghese is partly the imaginings of the Burgh Diaspora. This myth making reminds me of Benedict Anderson's book Imagined Communities:
In an anthropological spirit, then, I propose the following definition of the nation: it is an imagined political community - and imagined as both inherently limited and sovereign.
It is imagined because the members of even the smallest nation will never know most of their fellow-members, meet them, or even hear of them, yet in the minds of each lives the image of their communion... In fact, all communities larger than primordial villages of face-to-face contact (and perhaps even these) are imagined. Communities are to be distinguished, not by their falsity/genuineness, but by the style in which they are imagined.
Pittsburghese, the Steelers and Pittsburgh style sandwiches form the image of Yinzburgh Nation's communion. I suppose you could accuse Dr. Johnstone and this blog of romanticizing the Burgh Diaspora, but I think there is some solid foundation for claiming uniqueness. And thus ends my answer to Bill Toland's challenge.