What exactly are you wanting to know?
Shaman are Spiritual members of their communities. In the lore they are able to connect with the Spirits of their ancestors, as well as Nature(elements). They do this through multiple different means based on the Shaman in question’s culture. However often times this is done with herbs and elixirs and yes Totems. Shaman with a much stronger connection can achieve this connection through their own will. Thrall “was” such a shaman but even he was limited in his power.
On the subject of Thrall and Cataclysm he was asked and he hoped the Elementals could answer. Though for “reasons” most ignored him until he finally got an answer from the Fire Lord himself.
Lore wise Shaman don’t technically have any power of their own. True Shaman don’t and I’ll get to the other later. Shaman use “calls” to ask for aid from the Elemental Spirits within a given area. This aid can consist of but is not limited to physical aid or a buff to the Shaman or their allies. This is where we get abilities such as Lightning Shield, Earth Shield, and even Water Shield.
For the more offensive aid, Shaman become conduits of the elemental in question. Using their physical body to allow the elemental’s power to flow. If the Shaman is weak the power is throttled and becomes ineffective. When the Shaman is strong more of the elemental’s power is able to be released. However that doesn’t mean all of it will be. The Shaman relies on the Elemental’s abilities as well as their own. If an Elemental doesn’t want to give you their power then you’re 
. Think of it like your internet connection and Bandwidth. Broadband Vs Dialup. You could have a supercomputer but if your conn is poor or of low quality it will still be slow and unresponsive. On the opposite side, it doesn’t matter if you have a perfect connection if your system can’t handle the incoming data. It is also possible to overload a cpu or server causing it to crash. This is what happened to a certain Dark Iron who tried to summon the Lord of Fire. He succeeded but was killed as a result.
Now most of the above applies to true Shaman. As Zahirwrite mentioned there is also something known as a Dark Shaman. These were seen more openly within Garrosh’s Dark Horde, however have existed in the lore for a long time just under a different name. We call them Mages and Warlocks. Decay is one way Dark Shaman force their influence and will on the elements. Another is Arcane and Fel.
Dark Shaman are a corrupt version of what a true Shaman is. In some ways they are more powerful as they don’t ask the Elements but force them. Many people wrongly believe that a Mage’s Water Elemental is just a mindless construct. However Lore tells us that is not the case. a Mage doesn’t create their Water elemental with their arcane power, they summon and enslave it. The ones Jaina used on Org cried out and were heard by Thrall. This isn’t the only way Mages are akin to Dark Shaman either. All of their elemental magic comes from the Elementals. They are just oblivious to it even with all their arcane intellect. That or they just think themselves so superior they don’t care.
You saw me mention Fel above and Warlocks are not free of blame. Not only were the first Orc Warlocks prior Shaman, but many Human Warlocks started out as Mages themselves. Fel uses soul (spirit) magic and have been seen trying to manipulate the Elements with Fel. Gul’dan from Warlords of Draenor is even part of a quest chain where he is trying to do this very thing. Had we not intervened he would have succeeded.
Be it a True Shaman or a Dark one, their power often depends on if there are elements or spirits in a given area to “call” and their respect of the shaman or their power. This is why Thrall said his power was diminished when he first stepped through the Dark portal to AU Draenor. This is because the Elementals there did not recognize him or respect him as a Shaman, let alone the “World Shaman”. This would be true of any planet or plane of existence.
Having never actually done Argus I can’t be sure if the planet even had any Elementals that may not have otherwise been corrupted by Fel and the Burning Legion. Though as Warlocks are technically able to summon and control(contract) Demons who are part of the Burning Legion I don’t see why Shaman couldn’t have also garnered the respect of even such Elements.
As for Shadowlands… Well, our Lore has kind of been neglected or forgotten. As Elementals get sent to the Elemental Plain it is unlikely any would be within the Shadowlands unless summoned or brought along somehow.
“Enhancement” is mostly a gameplay spec but in-game it is listed as:
It isn’t hard to believe that in the case of this version of Shaman, takes a more direct approach to combat. Instead of channeling an elemental’s power through their own body, they instead use a talesmen of sorts. In the case above it is with Totems and the shaman’s very weapon. Though he is currently speced as Elemental; in RP(lore) Doseki’s axe is such a talesmen(totem). Forged within the Elemental plan itself it is an example of balance within the elements. Forged with Earth from the earthen plain within the fires of the Firelands, tempered with water from the plain of water and sharpened with lighting from the plain of air.
Hope I’ve been able to answer your questions lad. If ye have any other questions, feel free to hit me up.
