As a lifelong hardcore player I have a pretty similar build but I do have a few comments.
I think you had too much mana regeneration, 4 runes and 2 passives that are all about mana regeneration, I would switch at least 1 of the 6 if I were you. I think changing Spirit Walks rune to the life regeneration might be a safe option.
Another option would be to only drop 1 mana regeneration passives (probably Spiritual Attunement), and replace it with Pierce the Veil to use up all the extra mana you will have.
Also, I don't see the synergy with Horrify. First, it makes enemies spread out, and you have a decent amount of AOE. Second, the mana regeneration depends on fearing more enemies, and you will not want to be getting near large groups to get the most out of that rune. I would change Horrify to Mass Confusion (with the stun rune).
Finally, I think crowd control is more important than damage in HC, so I would change your rune on Grasp of the Dead to give more slow or reduce the cooldown.
Just some ideas, some of it is preference, but most of it is about increasing survivablitiy. Enough HC deaths over several years has conditioned me to go defensive by reflex.
Here is a sample build I came up with after altering yours. In this one I didn't switch the passive to increase damage, but did the other alterations I mentioned. http://us.battle.net/d3/en/calculator/witch-doctor#hZgjeU!bfV!caabcc