Let's start with Soul Shards.
If you ever played a warlock you should know by now that Soul Shards are the most annoying thing about the class, even after Turtle made them stack to 3. Earlier you were a looser that had no bagspace because you needed the shards to summon everyone to the raid entrance and often inside the instance. A summon slave that needs to go to raid entrance by themselves, farm shards and summon one person after another. Now? Well, you are still a looser divided by 3 lol. You don't really need a Soul Bag now, 10 to 15 stacks of shards is usually enough to summon the slackers.
Don't you dare running out of shards though. There is nothing more embarassing than that. Maybe you can cast curses in game but if you don't have your sack prepared, a powerful Big Boy 10 Generations Humiliation Curse Bomb will drop on your head. It will haunt you in your dreams and mages won't help because they are not real.
You can try to prevent that by going Affliction, also known as Excess Shard Generator (ESG). Here it works the other way around. You will be annoyed by your bagspace turning into literal hell (a place filled with bad souls). I heard you can go check up on a doctor and get an addon prescribed for that disorder.
All that playing around described above is for what? So you can summon people, your demons and different stones. But seriously, why is this a resource? Those things already cost mana. I could imagine a world where those things are free of Soul Shards. This is just an extra mechanic that adds NOTHING except frustration. People don't really realize that because they are brought up on this game and they don't question anything.
Demon mana now.
Gonna be quick on this one because this is a minor issue but still. So, why do demons have a mana bar? To prevent them from spamming their abilities indefinitely? Is this really the case? Imp can only cast its Firebolt, can't autoattack so in out of mana situation it just stands there... menacingly. Other demons have cooldowns on their abilities and they are often reactive abilities (Soothing Kiss, Devour Magic, Spell Lock).
Demon Mana adds nothing above what could be accomplished through cooldowns or balancing the abilities themselves. And balancing leaves a lot to be desired, lesser demons are so weak lategame almost everybody prefers to sacrifice them than play with a demon.
Basically, Warlock has too many resources to keep track of, unnecessarily.
- Mana
- Health (espeecially with Lifetaps and Hellfire)
- Warlock and Demon cooldowns
- Soul Shards
- Demon Mana
- Demon management
Warlock is already a very complex class and these systems are here to complicate it unnecesarily even more.
If the game aims to be complex for the sake of being complex then it's not a good design.