My response to the original question comes from a background of having been DMing HotDQ at Encounters at my FLGS for several months. We are in (I think) Chapter 7. And I apologize in advance for any spoilers.
First off, I'll reiterate the complaints about the imbalanced encounters and the difficulty with the dragon in Chapter 1, as well as the metagame cutscene fight with Cyanwrath. The player who faced Cyanwrath was a recent returning player to the hobby and he very nearly lost interest in the game. Cyanwrath's return fight in Chapter 3 was a TPK for both tables running it, and we allowed several "do-evers."
But my issues with HotDQ run deeper than encounter balance. Thankfully, I'm an experienced DM who can adjust encounters by increasing or decreasing the number of monsters or their abilities. In the example above, taking away Cyanwrath's breath weapon made the encounter merely difficult instead of nearly impossible.
The unforgivable failing of HotDQ, in my opinion, is its convoluted story arc. After (I think Chapter 3) the adventure gets very confusing and nonsensical. Trailing a caravan of cultists for months in game up and down the Sword Coast, through every major landmark in the region. Stopping at a Road House, which is a front for a secret tunnel, which leads to a swamp castle, which is a front for a teleportation circle, which leads to a hunting lodge, which is a front for a small town, which is a front for a castle .... It's all needlessly complex and anti-climactic. Important NPCs are brought up and cast aside at a moment's notice - never to return. No one's motivations are clear.
This is the main issue, and it's something that can't be excused by saying "oh, the authors didn't have the finalized rules." These are basic adventure design, basic storytelling rules that broken to the failure of the adventure path. This is worse than the imbalanced encounters. This is worse than the subpar maps. This is worse than the lack of proper awarding of treasure to keep the players motivated. All of those issues can be tinkered with ... but if the basic structure and plot of an adventure path doesn't make sense and information isn't relayed in a meaningful way, then the module doesn't do its job.
I'm not just arm-chair DMing this. I have written and published a better story arc than HotDQ.
http://rpg.drivethrustuff.com/product/3440/The-Coils-of-Set?it=1 If I can do this, as an amateur DM who did this as a hobby project, I'd expect better from the great Kobold Press, WotC, and their flagship Tyranny of Dragons campaign story.