I suppose the way an issue is illuminated by the photos is all part of what may make a press photo good.
I think what makes a pro is more about being in a place and/or having access to people (who you may have to interact with in the right way to get the shot) rather than superior technical skill.
You also have to put yourself in the position of sifting through thousands of photos - what stands out?
The New York Times link seems to have changed content.
This is the main website, and gosh what photos there are: http://www.worldpressphoto.org/
(I found it best to look at one winner and use next winner to carry on)
This really is a stunning array of photos.