1. This isn't a DH-exclusive problem. The anger w/ this particular issue should be directed at those responsible for class development, generally, for being very hit-and-miss with regards to what the players actually want from their class/specialization.
2. I don't take much issue with this, personally, because most of the "Next Class" proposals that remain are partially-implemented via other specializations (Necromancer w/ Demonology Warlock or Unholy DK, Ranger w/ Marksmanship Hunter, Buccaneer w/ Outlaw Rogue, etc.). There are things like Dragonsworn or Tinker that could have some unique take on rDPS, but ultimately both of them are still also melee-focused in their implementation.
3. Warning: You're about to experience a personal opinion -- I actually hope classes continue to be implemented w/ only 2 specializations. In fact, were it up to me, I think I'd actually chop up existing classes into 2-specialization classes as well, because for many classes the third specialization seems contrived and unrelated to the others.
4. Demonology Warlocks didn't really make sense as they were. I remain unconcerned with this complaint.
5. Their current standing, numerically, doesn't really reflect upon how fun they are/aren't.
6. All PvP sucks. Oh, we weren't just spouting our opinions as fact?