Didn't read 15 pages, so sorry if I am repeating other people.
Personally, I am pretty disappointed by this. In Heavensward, they reduced it to two with the promise of adding new and fresh content. I was super excited for this initially. The promises of Diadem in particular were so interested. I love exploration and I wanted to try new and fresh content.
That new and fresh content worked out to be: Diadem, Diadem 2.0, LoV, Aquapolis, Squadrons, Feast and Palace of the Dead (am I missing any?). I think that most can agree that Diadem, Diadem 2.0 and LoV were hyped up and all fell flat on their face very shortly after release.
Aquapolis is a bit more controversial, I know some people who love running these. However, I can't understand why or how, from my perspective it's beyond lazy content. The environment is incredibly dull and pretty much reused from old leveling dungeons, the mobs have 0 mechanics and provide 0 challenge. The only real 'game' is the 50/50 choice between choosing the right door. I can't understand how this is fun, in any way. The main benefit is that it gives good rewards.
Palace of the Dead is also controversial. As content, it's pretty boring and dull. That said, it offers a lot more than any of the content above. That said, it seems the only real reason people play it is for speed-leveling.
Squadrons were basically a copy of WoW's Garrisons, one of the most hated pieces of content added to WoW in it's 13 years of existence. Then, after adding it, they basically did nothing with it and it's pretty much dead content. While there is a chance to revive this content in SB, overall it seems like a poor idea.
Feast is arguably the best piece of content added. I personally hate PvP and never do it, so I have very little to add to the discussion. That said, for anyone who PvP's, Feast seems to have been a success.
So, there we have all of the new content. What about the content lost?
Well, we went from 3 dungeons to 2, but in addition to that, the 2 dungeons we got are lack luster. Overall the dungeons throughout HW were less satisfying. They were particularly bad for healers. Every single HW dungeon I was able to go in on release and let Eos solo heal it. I hadn't been able to do that at all between levels 31 and 59. It's just sad that they were tuned so easy to the point of boredom.
So, we lost a lot (1 dungeon and lesser quality on 2 dungeons) to gain dead content, repeatedly. This wasn't 1 flop of a dungeon, or 1 flop of a content. It was every single patch. The best thing I could have heard was that they were going back to 3.0 dungeons like in ARR, and will stick to what works.