Amazon possiblilty

Probably not the best location for this post.  I'm guessing you meant to put in the off topic?
 
I think a good thing.  I doubt it will change the cost of living much.  Pittsburgh is on a pretty steady incline of cost of living as it is.  I don't see it ever being on the same level as DC, NY, LA, SF or even Boston or Philadelphia.  One new company HQ isn't going to change it much.
 
Traffic would likely see a push, depending on where it is located.  My bet would be somewhere that 279 would serve as the major feeder.  If so, then traffic wouldn't be all that much more noticeable.  Most other roads being the primary artery, then likely would cause some major issues.  *IF in the city itself.  If in the surrounding area, there are plenty of places that could limit the impact, although would be a localized explosion.  I do believe if it went to Pgh or surrounding area, they'd have something in the package to add exits and widen certain roads to accommodate.
 
With how many mills and other steel supporting brown fields and other locations opened, I would be surprised if they were to wipe out much nature to make space for it.  Unless you are talking about those trees growing inside of the old JL/LTV mill in Hazelwood...  I'm sure most wouldn't mind that nature being taken away.  The housing stock in the area could easily handle 50k new residents.  Although, most likely half of those jobs would be filled by current residents.  There would definitely be a demand for new housing as well.  So, more of the suburban green space would take a hit.  Most of the green space in the city limits is either a park, a major challenge to build on or in a neighborhood that is so depressed, that building new is not a wise investment.
 
It would be nice for air travel.  20 years ago, you could get a nonstop flight to just about any major city in the US from PIT.  Now you have to fly through Detroit, Philly, Charlotte, Atlanta, Chicago and others.  There are more and more direct flights coming in, but usually seasonal, this would likely put PIT back in contention to be a hub.  The infrastructure is there, and Amazon's draw and tax revenue would allow for a much more competitive package for an airlines than they were able to offer when USAir took off.
 
It sounds like they are a top candidate, with many predicting it will happen.  Although, you can find predictions of all top 20 or 19 or whatever it is.  I would welcome it.

The only way Amazon comes to Pittsburgh, or anywhere for that matter, is if they have a reasonable guarantee of the facility not being targeted for union activity and the odds of that happening in Pittsburgh is not very good.

Take a look at where they placed their hubs in FL (between Tampa and Orlando — bear Lakeland for Steeler fans) and notice they are in low living cost, high unemployment (particularly for low skill or semi skilled labor), and non union areas. This is to provide a relatively cheap labor pool that is willing to work for what they are offering and be happy about it because it is leaps and bounds better than their current condition.

Great point on the union aspect.

If this were an automobile plant or some sort of manufacturing, then union would definitely be a major consideration.  Very likely a reason why manufacturing never came in and took advantage of the saturated skilled labor supply that Pgh had in the 80s.

We have enough traffic already

And, in no surprise to any one, it is NYC and northern VA.