The emphasis on PvE is the main bit - I'd argue that you can fully experience and enjoy Hearthstone Mercenaries without encountering another player at all. It's got plenty of differences to be fair. So, how does Mercenaries do in comparison to Battlegrounds, the other Hearthstone-adjacent experience that evolved like a mutated Murloc out of the marsh? There's also a PvP (player vs player) mode you can unlock later on. That's alongside roguelike elements making each 'bounty' go slightly differently each time. This time around, it focuses on Pokémon-style battle scenarios, with teams selecting moves to use on each other and different speeds determining attack order. Hearthstone Mercenaries is the latest addition to this brand of 'entirely new game but it's still Hearthstone' design philosophy that worked extremely well for the massively popular Battlegrounds. Some areas are the same (health and attack values, keywords, characters), but the actual way you play is completely different.
It's essentially a whole new game though. Hearthstone Battlegrounds took the auto-battler formula popularised by DOTA Autochess and Teamfight Tactics and brought in its own Hearthstone-style flair - the World of Warcraft universe crossed with an off-kilter, knowing sense of humour.
