Death Knights are based around a four resource system: Runes, Runic Power, Diseases, and Time. Each of these resources changes differently based on a miss. A miss of a Rune does not cost that Rune (it regenerates in less than the minimum possible GCD). A miss of a Runic Power does cost that RP however. A miss of a Disease can not only cost that disease, but cost an additional amount as diseases are required, most of the time, for effective dps. Finally, a loss of the time to cast the spell, as well as additional time recasting. Because rotations are often times dense and diseases can run out, this loss of time can have a heavy impact on the Death Knight.