Shamans are the Horde's very aggressive answer to the Alliance's very defensive unique support unit. In Legionnaire+, there are no factions, meaning Shamans and Paladins can team up together. In Classic, the Shaman was known for its straightforward playstyle, and very explosive healing and damage. Shamans burned bright, using their entire mana pool in a few seconds to keep a key target alive or burst someone down with Chain Lightning. Elemental was very clearly their best archetype, with Resto struggling to have an identity and Enhance being a jokey, fragile, inconsistent RNG spec.
In Legionnaire+, we gave a lot of love to Enhancement. Their core identity as a deadly slot machine is still in tact, while giving them some extra tools to compensate for their clear and obvious weaknesses. Their Ghost Wolf form is now instant and Rehgar's Fury is a new short-range gap closer that requires you to be in Ghost Wolf form, charging towards your target and dealing weapon damage. In addition, as a callback to Alpha WoW, they are able to purge targets even if they have no magic buffs, enabling Shamans to put targets in combat instantly from a very large range. If they -do- remove a buff, their next Shock is more powerful and they recast a Lightning Shield for free. Finally, they've received a 0.5 second global cooldown reduction on their totem casts, letting them "enhance" their party members quicker and enabling them to get into the fray sooner.
Restoration in Classic was mainly a PvE spec. Mana Tide, their capstone talent, has 5 hp and a big obvious animation that makes it a clear priority target during skirmishes. That, coupled with the fact that Elemental Shamans could heal almost as well in a PvP context, means Restoration really never got a chance to shine. We've moved Mana Tide totem up higher in the tree and given them Earth Shield from later versions of the game, allowing them to chose one priority target on their team to passively heal. We've also removed Chain Heal from the base kit of Shamans in order to give Restoration more of a clear identity. Chain Heal heals about as much as Lesser Healing Wave and costs a comparable amount of mana, but heals other party members and has a 12 second cooldown. We hope this gives them some staying power when put up against other healers.
Elemental Shamans did not change as that is the meta spec.