For the duration of the fight, creatures in the immediate area will not be visible to you (but they may still attack you if they wander too close). Keep in mind that other players will still be able to see you and attack you if you are eligible for player vs. player (PVP). Other players will also be able to view your pet battle in real time, including the pets involved as well as their health as the match progresses.
The original WoW was painful. Mobs took forever to die; one additional enemy in a fight was a pain, two probably meant death. There was a ton of running. Most buffs lasted two minutes, many took reagents, abilities were trained and often out of reach if you lacked the necessary gold. Warlocks had to farm shards, hunters had to carry ammo -- even my warrior did, since in vanilla WoW she can carry a bow and fire arrows herself.
Instead of sharding the zones, their plan is to shard the entire continent at launch. In Classic WoW, there are no dynamic spawns and questing in starting zones would quickly become annoying, so it's understandable that the game is all but fun when there are no mobs to kill, but layering will also cause you to see new players, every time you sign in to the game, which is a clear downside of the tech.
Things like Tauren melee reach being bigger, which is something I experienced going away live during Black Temple Illidan progression (at least for that particular encounter), is confusing to players as it's a significant advantage over other races in some instances. No quest tracking is another apparently confusing issue, as many Mankirk's WIfe seekers well know, and there's a whole lot more where that came from:
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