Originally Posted by
exochaft
I think part of the reason for excessive mob density is increased player density in specific areas of zones and a side effect of flying. In Classic, even one extra player in a questing area can be enough to completely slow down your questing experience. The response to this issue over the years has been things such as non-party shared tapping of mobs, phasing/layering, and (specific to this point) increased mob density and respawn rates. Now how does flying affect this aspect of the game?
Well, you can go back to Classic to understand how this evolved, as well. In Classic, the world is very big in comparison to your ability to navigate said world, which leads to players being spread out quite a big... and even then, as mentioned before, you could have even one player adversely affecting your gameplay. Now, add flying to Classic areas, and what happens? The world instantly shrinks quite a bit from a travel time perspective, and you suddenly have a potentially larger and more frequent fellow player intrusion into your experience.
In comparison, let's use Nazjatar as the counter-example, as it's a relatively small landmass that is one of two centerpieces for most outdoor activity for a patch cycle. The area is so tiny that it barely takes any time to traverse even with ground mounts, so how do you counter or rather slow down constant player intrusion? You make the world more dangerous to force players to slow down, and this is done through scaling up mobs, creating difficult-to-traverse terrain, and increasing mob density... and this difficulty jump is also a side-effect of player/gear scaling when the content is new, which is a whole other topic. Once you add flying to the area, you've just made the player intrusion probably even worse again as you've shrunk an already small area even more and negated the terrain as a method of slowing down players. This requires one to likely tune mob density/difficulty even higher due to increase traffic rates.
So what's the big take-away? Well, the biggest one is that the world is tuned and designed to have flying nowadays, so when you don't have flying it's going to potentially feel longer and more arduous when you don't have flying. However, that's an experience in itself, and any sane person would likely appreciate flying more after experiencing the content w/o it versus having it day one. Player feelings aside, flying itself allows players to easily bypass almost all difficulty in the game in terms of the outdoor dangers, as well as circumventing triggers and content/experiences that can only be had when confined to the ground. While there are some solutions to allow flying at the start, the experience it creates for the players doesn't even come close to what can be had w/o flying, and generally the solutions are cumbersome at best. One of the main reasons why Blizz regrets adding flying is because it's very hard to design content and an interactive world if the players can just skip it all, because players will tend to take the path of least resistance.
The best way one can view what flying actually is in WoW is a perk for "playing through the game once." What does this mean? With how flying is achieved, all you have to do to get flying is play through the campaign and explore a bit, and you'll naturally gain the achievement for flying. Once you get flying, it's akin to the dialogue option that says "I've seen this before... <skip>", as it lets you bypass and/or speed up traveling that was part of your initial experience. Why is it tied to an achievement? Because achievements and their progress are easily tracked by a player, and it's an easy vehicle to make "playing through the game once" give you an account-wide perk.