I've leveled a few DH's and through NM/Hell (not through choice!!) and only thing imo that is essential is Preperation/Battle Scars. You can afford to play loose and fast with everything safe in the knowledge that you have 60% health and at least 30 disc on tap and that gets you out of pretty much anything, up till Inferno..
If disc management is an issue, swap Vault to Rain of Vengeance - Stampede. The reason Cluster Arrow is so good for heavy kiting situations is because you can use it to dump your Hatred within a few globals. Proper stutter stepping technique is the cornerstone of ranged play, and the justification for defensive stats is that while you should be avoiding hits at all costs, you should at least be able to take quite a few hits in tight situations. For heavy kiting situations, I like instant cast spells like Cluster Arrow. You may use the Cluster Bomb rune if it turns out that does more damage for you for your play style. For group play, when the mobs are mostly glued to the Barbs/Monks, I have found NT to be more effective than Ball Lightning. I believe Ball Lightning iterates 155% weapon damage over 2 pulses, whereas NT is 155% over 1 pulse. Regardless, Ball Lightning just feels weaker upon tests.
My Inferno entry stats are as follows:
I'm pretty happy with my stats right now, although I would like to achieve more armor and close to 600 All Res. That should make Act 1 farming a breeze. At the moment, I am going back to my roots and leveling a Monk waiting for the farming group to catch up. Have an Act 1/2 viable Wizard, but again, group is dead :(
Friend's lvl 60 DH died yesterday in a party of 3. Teleporting, nightmare, arcane, something frogs. A frog teleported behind the DH and instantly cast nightmare which force the DH to walk into 4 arcane beams. Death in less than half a second. Admittedly, he had a bit of a glass cannon build (partly due to playing with a monk tank), but still - there's just no way around that kind of death without some really significant resistances and hp...
I use basically the same build as Simfish, with a couple minor changes: I swap between HA/DA and Evasive Fire/Covering Fire depending on area, and I prefer Ball Lightning to NT, but that's just a personal preference thing.
The primary difference is that I much prefer to use Rain of Vengeance/Stampede instead of Vault (which she does suggest as an alternative). One huge benefit of Stampede that is often overlooked is that it is tremendously good at separating Elites from invulnerable minion packs. Just target the pack as a whole when you've got some space between you and them. As the pack moves into the AoE the minions charge right through while the Boss gets knocked back several yards in the opposite direction. With their lower HP (generally), it gives you enough time to kill the Boss before the minions find their way back... or at least inflict serious damage. This is, hands down, my favorite way to handle invulnerable minions.
that's my friend's DH tank on hardcore. Works in a1/a2. For a3 still needs better gear tho.
Spike trap is procing LoH like crazy.
CC + Melee Attack + Combo = Dead in Act 3 against Swift Skull Cleavers and Tremors with those stats. I would be terrified to play a tank DH in HC. The Act 2 elites do like no melee damage, but some of the Act 3 mobs can legit global or 2 shot you with just their basic melee attacks. My SC DH had way better mitigation and LoH, but none of that fancy set bonus stuff, and got stomped hardcore in really clutch situations. True, it is very possible to progress if you play very well, but one lag spike and you're out.
Your friend does plenty of damage. Add a Barb to the party, get an additional 200+ All Res, and I don't see why he can't beat the game from here on out.