Eureka Orthos, while being basically a carbon copy of HoH (which in turn is a carbon copy of PotD), still has enough differences from PotD and HoH to talk about that reveal why deep dungeon was a successful concept back in HW (that sadly has not seen innovation). Anyone who soloed both types would understand.
With talk of SE making a new deep dungeon that will supposedly innovate much more than what we've seen so far, I hope that SE remembers what make the original deep dungeons fun for hardcore players (people who clear it), and these differences between PotD+HoH and Orthos reveal much.
The biggest departures of Orthos from PotD/HoH are that:
(1) Monster and boss mechanics are way more dangerous
(2) Monster autos are much lower
However, there is a slight consensus that Orthos is less fun than HoH, and far less challenging. That seems unintuitive: who cares about autos? Yet this makes all the difference.
While I liked that (1) made bosses much less of a snoozefest and made monsters more dynamic, (2) actually hurt the identity of deep dungeon in Eureka Orthos. That's because a huge draw of PotD/HoH for clearers is the necessity to carefully manage pomanders and manage RNG. With Orthos, that is practically non-existent unless you were playing on Astrologian. That's because on any DPS job, if you successfully execute the mechanics for the monsters and the bosses, you can clear very reliably and easily:
Time constraints and incoming damage are not an issue on DPS and tanks in Orthos, since they can kill any monster very quickly while not receiving much damage from the monsters at all. Formerly, DPS killed monsters quickly in PotD and HoH but are much squishier and necessitated kiting and optimal usage of pomanders of steel, along with a good knowledge of the outgoing damage from every mob in the higher floors. This required clearers to be strategic in situations that demand a choice from the players. For example, if you saw a treasure room full of Flood Dragons and Chimeras, what do you do? Do you try to pick off the proximity mobs (Chimeras) who don't deal a lot of damage since they are constantly casting, and try to sneak pass the sight mobs (Flood Dragons) when they aren't looking? To do that will require you to expend some time killing Chimeras who have much higher HP than other mobs in that set. Or do you want to use Rage to kill the treasure room to expedite the process? But what if you only had 1 rage left, or what if you have 16 mins left in the set? Or if you aren't getting any Steel drops, do you want to risk opening gold chests and get a Mimic, or should you just move on and pick off the archaeosaurs? Do you want to use Serenity on this floor with a Gloom and Amnesia debuff or use Rage through it, or simply power through it? What if it was flighted or altered? These are the many decisions players need to make in PotD and HoH. Due to how rare gold chests are beyond 180 and how dangerous everything in 81-99 is in HoH, it's fair to say that optimal pomander usage is essential to get a clear if you're solo.
That isn't the case at all in Eureka Orthos. As long as you can do the mechanics associated with every mob, you can kill them without worrying about Steel. Floor debuffs largely have no bite other than No Items, because mobs don't auto you hard enough for Gloom to be noticeable. You can pretty much randomly use pomanders and get a clear in Orthos as long as you can do the mechanics. Autos not mattering in Orthos seem like a small issue, but the way it interacts with the rest of the system reveals why those hard-hitting autos made PotD/HoH a different kind of skill check. It forces DPS players to understand every mob and their outgoing damage and therefore have to make snap decisions on which mobs to pull and which not to, depending on the floor layout, the patrols, the floor debuffs, and the current pomanders held. This blends a strategic layer to the gameplay: to plan out your pomander usages while adjusting on the fly to unexpected situations tactically while going through the floors. In Orthos, the only thing required is the tactics: resolve mob mechanics and it's a free win.
Part of what makes deep dungeon enjoyable is that it's a departure from how most other combat encounters in FF14 work. It requires the player to think strategically about multiple interwoven systems: pomanders, traps, floor layout, DD mechanics (such as patrols, mimic rates), and of course the actual combat. Orthos reduces that to just performing the combat mechanics correctly. It's a huge shame.
Edit:
What should be kept from Orthos, and the good things that Orthos did:
(1) Bosses that have actual bite, and do actual mechanics that are arguably about as difficult as the easier duels in Bozja. This is good.
(2) Mob mechanics are dangerous and many one-shot you. I don't like that a lot of it boils down to memorizing a bestiary, but some of these mechanics make the moment-to-moment gameplay far more dynamic. Some people hate it, but I particularly enjoyed the mechanics of Orthos Fitter and Orthos Sasquatch. They do a huge pointblank AoE with an extremely late telegraph that will one shot you even when out of combat. While in PotD and HoH some mobs also were a nuisance when out of combat, these actually demanded proper respect. More mobs should be like these two, but not necessarily simple one-shots. For example, a mob that slowly walks around the floor and eats other monsters and getting a huge damage buff, meaning that you may want to find it ASAP or stay out of its way. Or a mob that starts homing in on you whenever you attack another enemy and explodes if it gets to you. Also, while I like mechanics actually being dangerous for once, having everything from 71 and up basically be one-shots is a bit too much; I'd rather have more difficult mechanics with possible recovery states.