Funny to see all this spilled ink on Pittsburgh (both here and across other media) when there is another city in the same state that may be the single best choice in the whole country.
Philadelphia is a major city in its own right and perfectly located in the middle of the Northeast Corridor, with incredibly easy access to other BosWash cities. An HQ there would be able to draw talent from across the northeast, from Boston to Pittsburgh to DC, not to mention the universities already in its orbit (UPenn, Drexel, Temple, Swarthmore, Villanova, Bryn Mawr, arguably Princeton and U Delaware). Culturally it's in the same league as Boston, DC, and Chicago and way ahead of an Austin or Denver, with a diverse population and growing edgy/hipster vibe that would appeal to recent grads. SEPTA isn't amazing but is a pretty damn solid subway+commuter rail system by American standards, and connects Center City to a major international airport. Finally, relative to all it has to offer, Philly is still DIRT CHEAP!
So there you go, why wouldn't Amazon pick Philadelphia? It makes so much sense and so many different levels. Better than Boston, I'm afraid (too expensive), and way better than Pittsburgh.