My suspicion is the Gargoyle is OP at the moment. It may only be in situations where the DK needs a lot of burst on demand, but it definitely seems more popular than we suspected it would be.
Heart Strike should be your go to ability in most cases. When you are at risk for breaking CC, you can use Blood Strike instead. But the intent is that Heart Strike will do more damage. It competes with Frost Strike and Scourge Strike in the other trees.
For PvE, we've seen some pretty decent numbers come out of Blood. I think players just need more time to try out all the build combinations. For PvP the big hits of Scourge Strike, the mitigation of Bone Shield and AMZ and the extra damage from the Gargoyle and ghoul probably make Unholy overshadow the other trees. This should be better next patch.