It seems like the warlocks are only losing stuff with every expansion instead of gaining in terms of power, lore and overall aesthetics, some of their stuff that remained are not integrated into the main warlock class system such as the talents or spellbook but are hidden far away, here are some examples.
In the past we used to be able to have the grimoire of supremacy which upgraded all demons into other stronger versions but it was removed from the talent tree and instead turned into glyphs for each seperate demon, and because the glyph system was also removed from the main class spellbook, someone has to go and discover that there are glyphs that you must buy and change each demon, and you must buy them seperately for each spec too, so not only it became far less accessible but the average warlock will not even know about those glyphs, but with the talent grimoire of supremacy everyone could see it.
This is possibly the biggest downgrade of all, one of the final talents for warlocks, was able to make the inferno and doomguard as permanent minions, and in addition with the grimoire of supremacy, we were able to turn them into abyssal and terrorguard, which was the peak of the warlock class lorewise, but after both the talent, grimoire and glyph for upgrading them were removed, now there is nothing left for that other than a few seconds of being able to call the inferno, and the doomguard went all the way back into requiring a whole ritual of other people which almost nobody uses, that was the worst decision.
The main warlock abilities for each spec, shadow bolt, incinerate and malefic grasp used to be able to be casted while moving around, but now none of them can do that from either the passive talent or by class design, you have to be standing still to cast them, and in addition, the malefic grasp ability of affliction warlocks stopped being the default spell, and instead they retreated back to drain life which visual effect is underwhelming in comparison.
The green fire questline and the codex of xerath which changes the fire spells into green, is possibly the most major class enchantement of all, yet instead of warlocks either passively having green fire by default to make them different from mages, or at least for the codex of xerath and the questline being unlocked automatically from the spellbook, the only way for someone to learn about it is to either to see another warlock using green fire and asking them how they got it, or to come across the information randomly and read about it, it is so inaccessible to the average warlock that it is ridiculous for something so important.
This might be a downgrade for all classes but especially for the warlock, the loss of the artifact weapons, they contributed very highly to the identity of the class and for those who used the inferno or abyssal as permanent minion, they strengthened it as the greatest tanking companion and they were also making it able to summon more of them when you called it for the first time, this along with other important enchantements that came with the artifact weapons, now the only thing that remains is the transmog of the weapon which feels kind of hollow.
There have never been any further permanent demon minions with any expansion and there is not even an extra spellbook list that we can see all the demons that are available there like the hunters have for their pets, but they are just scattered around all over, in the green fire questline we could tame a pit lord which was even above the abyssal and terrorguard but it was temporary and only for that questline, now all those are gone.
Some of the important passive abilities in the spellbook were removed from each spec, some were removed completely but some were made invisible which confuses the average warlock even more, for example destruction warlocks used to have a passive called Chaotic Energy which increased their mana regen by 625% and spell haste also increased mana regen, that passive was either removed or made invisible which is bad either way, especially for a warlock that was not here when it was still in the spellbook.
The early tier sets and pvp sets for warlocks used to be much more tied to the class and theme of the warlock, especially the tier 6 set and vengeful gladiator set which had the warlock wings when you jumped and when casted some specific abilities, although the helm looked bad because of the hair becoming invisible issue with wow and the wings should have been tied to the shoulders rather than the helm, but all of those sets were adding a lot more to the warlock identity, while the newer tier sets look like bland empty hollow paladin scraps with no identity to each class, especially for the warlock.
The demonology warlock had their metamorphosis transformation removed because the demon hunters had to steal them, and there are probably more but those here are the most significant changes that made the warlocks go backwards.