Diference Connecting AC/DC
AC supply 

Is a periodic function like a sinusoidal wave. It goes both +Ve and -Ve at regular intervals of time. So if we even interchange the supply terminals, it would just make everything negative i.e. +Ve becomes -Ve while -Ve again becomes +Ve and hence both the RMS value and average value of measurable quantities remains same over the time of measurement which is generally quite high as compared to the time period of one cycle.

Also it will matter if the AC Supply is not symmetric for the periods when it is +Ve and -Ve, even then interchanging the supply terminals will make a difference.

DC Supply

The supply is constant hence the potential of one supply terminal is at some constant difference from the other so interchanging the supply terminal will change the new supply to negative of the original one, hence changing all voltage and current parameters to -Ve.