Contrary to popular belief it has nothing to do with hatred generation. In fact I'd go as far as to say that hatred generation with DH is currently fine as it is. It's possible for DH's to hit upwards of 15 hatred regeneration if they gear in that manner, yes, it does reduce sheet dps (which most DH's don't want to give up) but it solve some of the hatred issues that most people raise.
However, my analysis is delving into discipline generation and discipline spenders and I feel that Blizz has taken a really bad approach in balancing the DH class.
For one, discipline has one problem in that it is only used for Gloom (and occasionally vault). It could be due to that Gloom is too overpowered a skill that it's the only discipline skill that you need to spam, or it could be that Gloom is the only viable discipline spender given the current state of discipline generation.(discipline generation is too low).
Nightstalker as it is is actually pretty fine but it does need a small buff. I don't exactly know % chance nightstalker has to proc. But I would say that it needs to proc about 75-100% more.
Discipline shouldn't be viewed just solely as a "defensive resource". Players should be given the option to utilize it offensively allowing more variations in playstyles. Skill's like Preparation:Punishment have the right idea in mind, but have really poor execution.
However, doing this would mean reversing a good deal of skill that blizzard has nerfed over the duration of the game.
For one the trail of cinders nerf should be reversed. ToC should deal at least 500-700% damage.
Jagged spikes should be given a proc rate of 0.1.
Smoke Screen Choking gas should have an AOE increase to about 10 yards-15 yards.
Will add in more as I keep testing.
My DH is currently doing MP10 fields and does just fine. (though the lag is sometimes annoying)