Simulating multipath channel
You put zeros in between to show delay between the channel taps. I don't know what $M$ is in your code but suppose $M=10$ and say your sample rate is 10 MHz. Then you can interpret mc = [1 zeros(1, M) 0.28 zeros(1, 2.3*M) 0.11];  as you get the first multipath component with zero delay (gain = 1), then 10 zeros later (which is equal to 1 microsecond) you get another multipath component (gain = 0.28), and finally after another 2.8 microseconds you receive the last multipath component (gain = 0.11). 
It looks like the "mild" vs "harsh" is just language that the author uses to let you know that mild happens to rarely cause errors where the harsh causes many errors.