My main concern, if Amazon came to the DC area, was that the company would head out to the perimeter, build another suburban office park campus, and exacerbate sprawl. Glad they didn't do that.
From a DC standpoint, Crystal City/Pentagon City/Potomac Yards is a major commitment to the Orange and Yellow Lines. The commuter roads would be I-395 and U.S. 1, in the heart of one of the nation's worst traffic nightmares. I hope Amazon will discourage driving. It could do this by refusing to pay for parking and restricting the number of parking places provided to employees. If there is a company transit subsidy to employees, it should go impartially to employees regardless of their mode. Employees who live close enough to walk or bike could pocket the money.
Infrastructure improvements will be part of the deal. While I'm opposed to corporate subsidies, improvements to general use infrastructure are appropriate. Metrorail is finally dealing with a huge deferred maintenance problem. That has to stay on track. I presume the proposed Potomac Yards metro stop, which is much needed, will be accelerated. Metrorail needs to increase rush hour capacity and I hope this is part of the deal. Increased park and ride capacity, especially at the more distant suburban metro stations, is also needed. Metro should be doing these things anyhow. If Amazon coming to Crystal City cuts through the bureaucratic and political gridlock, good.
The upper Yellow line runs through some now-gentrifying areas in north and northeastern DC. It currently ends at Fort Totten. I suppose it could be extended. Of the various Metrorail corridors involved, this is the one that would provide direct (i.e., no-transfer) transit from still affordable housing to the Amazon complex. The other ends of the Blue and Yellow lines run through pricier areas.
On the Yellow line, U-Street/Cardozo and Columbia Heights are already major gentrification success stories. Shaw, Petworth and Fort Totten are about to get a lot more interesting. Commuters on Red, Orange and Silver lines can always transfer, but that's a time-consuming hassle. The Blue line will go directly to the Amazon complex, but Blue from the eastside involves a long U-shaped circuit through downtown. There are twelve metro stops on Blue between E'Enfant Plaza and Reagan Airport. Yellow is going to be the hot Amazon commuter line.