You're on a retina-equipped Mac. The change was done on purpose, to fully support your high density display.
Now when you view images at 100% zoom, it literally is using each pixel of your display to show one pixel from your image.
Earlier Photoshop versions were running in pixel-doubling mode - using 4 pixels (2 by 2) on your display to show one pixel in your image at 100% zoom.
If you want to view it as it was before, try 200% zoom.
Thanks, Noel. That will take some getting used to because I don't remember there being as much pixelation before, which now seems much more noticeable at 200% zoom now.
I haven't personally seen a Mac with a Retina display run an application in pixel-doubling mode. It sounds as if it may do more than simple digital duplication of pixels, but rather some kind of interpolation, which would look smoother.