Workaround:
I changed grid scale to either 100 or 50 first to start with, then created a new Empty Level, I then changed the scale to a lower number. NOTE: The editor maintains the grid scale, so if you lowered the grid scale (to such as 10, 5, or 1) and create a new Empty Level you will probably find the same problem as before.
Possible Explanation:
The lines may be very very hard to see, it is even on my 46 inch screen, you can right click rotate the mouse down slowly and see the grid below you. The grid render quality has been increased so the lines appear thinner and shader had either been adjusted or replaced for better performance somewhat like Tim Sweeney said.
I found this on the problem here: