Death Strike is a bit of a strange skill in the Death Knight arsenal. Leveling, you use it in place of Scourge Strike/Obliterate when you need healing. In a raid, likewise, although you may be significantly less likely to need healing. In patch 3.1 this changed a bit, with Death Strike becoming the default UF ability for Blood builds, whether or not they needed healing at the time.
However, this beefed-up Death Strike for Blood meant that Blood players were getting huge heals from DS without having to pay a damage penalty, which was an issue both in PvP and, apparently, while tanking. So after a brief period of being hotfixed to provide an unspecified amount less healing, DS now heals for a flat 5% of the DK's health per disease on the target, up to a maximum of 15%. The amount healed is no longer based on how much damage Death Strike does.
Among other things, this means Unholy DKs, with their third disease, will actually get more healing from DS than Blood DKs who have talents to boost DS's damage. I'm not sure this is a problem, especially given Blood's other health-generating abilities, but it does feel slightly strange. I haven't played Blood since early leveling, though - do current Blood players have any opinions on the change?