I think it suffers from bandwagon hate. 3 people hate it so I need to hate it to. Yes it had issues with balance and magic item distribution but it was made before the monster manual and dmg were done. The other is DMs running it poorly. The first town invasion is a sandbox with a timeline but I see people running it linear and forcing all the interactions, then complaining. But, I can see meta players not liking some of the scenes where you are fighting something more powerful but in a boss round puzzle way instead of a straight fight or your setup to loose. Steve Winters had a great post somewhere about some of the changes he would have made in hindsight or bad edits on wotc part.
I agree some of the issues can be ascribed to the fact that it was the first adventure module. The first adventure is often either kind of rough, or else so heavily polished by repeated playtesting that it shines. HotDQ isn't the one that was polished. LMoP was.
Further, I agree that it being released
months before the DMG was significant, too. It was released well before the 5e encounter guidelines even existed. I think that threw a lot of people off. I almost wonder if Crawford took feedback from HotDQ and used it to help figure out how to peg the encounter difficulty to get the gameplay pacing they were shooting for.
Third, IMO, it was an adventure path that didn't have clear module-like breaks. At least it didn't when I was playing it. It felt constant until the end of RoT. Maybe that was our DM, but I'm not sure. I have been a big fan of adventure paths that are packaged as a series of modules covering 1-4 levels because then it feels like a series of contained adventures with meaningful breaks. Age of Worms does this pretty well. So does GDQ and most of the AD&D module "chains". HotDQ and RoT didn't feel that way to me. When you don't do that, like with Return to the Temple of Elemental Evil, you can end up with a
massive dungeon slog, or a section requiring a significant amount of prep work for the DM, or just feeling like there are no breaks in the story at all. HotDQ's pacing felt off, but I haven't run the adventure myself, only played it, and it's been a long time since that. I just remember wanting to have breaks for downtime and not feeling like we could have them.