Seems like those who have been in PVP battles against groups of multiple warlocks, then felt the sting of Death Coil landing and sending you screaming off into the distance have something to look forward to in their upcoming battles... somewhat. According to Eyonix on the General Forums:
We're actually going to be updating the patch notes soon, with a new note indicating that Death Coil will now be subject to diminishing returns with itself, in the Horror category.
Currently, only Death Coil is in this category.
On the other hand, Death Coil is on, what, a 2 minute cool down? As such, this is really only going to apply if you get into a fight with more than one warlock Death Coiling you at a time.
Perhaps it's just me, but it doesn't seem like this is going to be too terribly much of a concern to most warlocks running around on their own. (I admit, my 'lock is only 58) Especially when you consider that Death Coil's fear effect doesn't diminish based on a warlock using other fear effects -- only itself, currently.
Now, if you make it's effect diminish when combined with other fear effects...