Flying makes all of the outdoor level design irrelevant. Why even put mobs there if you can mount, fly past them, herb/mine/quest, mount and fly away. There is something to be said about having to fight your way through them or try to avoid them. It makes the game MUCH more interesting. Otherwise every zone ends up feeling the same. Also, yeah it sucks if they kill you or daze you off your mount. Be better, get more gear, or avoid them then.
Flying should be used only to bypass old content when it doesn't matter anymore but you need to get through it. Or *maybe* to make it faster for alts. It still will trivialize the content on alts though so still not sure how I feel about that.
All *current* content should be ground mount only or it honestly doesn't matter how they bother to design it. May as well just put every quest mob and node on a flat circle and just have you aoe them all down and you can turn in every quest at once and hit 100 in a minute.
Also, just because it may take longer to finish the action (quest, farm, mine, etc) it doesn't mean that it will be less efficient overall. If it takes 5x longer to complete a quest, it can give 5x the exp or reward. I remember in Vanilla spending quite some time on a quest and its reward was WAAAAAY worth it. I'd rather have 1/10th the amount of quests that each take 10x more time to do and offer 10x the reward. It makes doing them feel like it was worth it and they can be better designed and much less repetitive.
Also, yes flying would be useful on Timeless Isle. But then none of the mobs or pvp or any events would even matter. You'd just fly to each one, never miss one, and it wouldn't even require effort because everyone could get there instantly and just burn it down and fly to the next.
So, as far as I'm concerned, flying should be restricted to a convenience to hurry through outdated content. It basically ruins all the design effort put into anything.