Is mining difficulty leveling out (Spring 2015)? How does this affect the economics of Bitcoin?
It's pretty easy.
The first asics where based on 90's technology, current asics are almost cought up now to recent chip technology.

So the asic industry has camped 15 years of tech advancement into 2 years.
Every couple of months the new asics where more than twice as fast. That caused the rapid hash increase.

Now that they are caught up with current tech we will go back to 10% efficienty increase per year again. Like in the gpu days. Back then the difficulty raised very slowely because gpu's also had a 10% efficienty increase per year on avarage.
When gpu's came to the scene you saw the same rapid growth that flattned again when everybody had switched to gpu's. The only reason it was just a short period is because retail gpu's where allready in line with the chiptechnology of the time. 

Now that asic efficienty improvements are in line with the industry the only way to increase hashpower is to have more machines instead of ever faster machines.

All that you are seeing is growth in hashes per watt flattning out, and with it difficulty growth flattning.