While it is nice and all that they are doing this to encourage PvP and player interaction, I still feel that it would been easier to make changes to the flying mechanic so you do not need to put restrictions like this.
I mean, flying is fine in my book, but if it is vastly superior to ground mounts in every single way, then there is a balance problem, as I see it.
You could always just increase the casting time for getting on a flying mount or at least make sure it can not get up in the air immediately by just pressing spacebar.
Perhaps you could even restrict flying on mounts for a period of time (like a few seconds) when mounting up and then you could take up to the air? At least then it will not be so easy to just get away in a whim while at the same time not making flying useless.