I have played this game since it came out, with occasional breaks, and the best mounted movement has felt to me has been during Dragonflight/MoP Remix. The ability to choose between between two things on the fly contributes to this.
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Slower precise flying for someplace hard to navigate while flying, with the bonus of being able to go afk on a whim when real life requires it, without having to search for someplace safe to land.
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Faster, more involved flying for when I want to move someplace fast or feel much more immersed in the game itself, and a much greater method of crossing several zones while afk.
I think being able to do both by only a 1.5 cast time to swap between mounts allows the ability to play the game at a pace that felt as freeing as it did immersive. I only speak for myself, but I swap between the two methods of flight constantly, and this new change severely cuts into the amount of time it takes to do absolutely anything in the open world.
Now, if I want to change how I am flying, it takes a lot more effort. First I have to land, then dismount and cast Switch Flight Style, and then resummon my mount. This takes 6.5 seconds. 5 whole more seconds to change flight style for really, no apparently good reason.
This is not to say I dont like many mounts having the new flight type, I think it is a massive improvement to the game, but why not make the spell a 1.5 second cast that can be done while mounted, but still/landed. Retaining the same exact timing it would take to swap flight styles as before. The change feels like a punishment compared to its previous iteration.
I hope others agree with my sentiment and urge blizzard to shorten the time it takes to swap between flight methods.
EDIT: There’s been loads of good suggestions, outside of just shortening the cast time. (Which, imo is the weakest of the alternatives.)
Instant swapping.
Swapping while flying using aerial halt.
Instant but dismounts, allowing for the same time to swap.
Set individual mounts to what style you choose.
Set individual mounts to a style, which those outside of that default to your chosen flight style.
Set individual mounts to a style using mount equipment, the rest default to your chosen flight style.
Instant cast toggle, but swap mounts/remount to take effect. Or take effect when landing.
Make Dynamic Flying slots in the mount tab like Battle Pet slots. (Or the reverse way, slots for static flying)
(I’ll add if I missed some, or if there are more suggestions)