My two pence, I hate the idea of no flying and it really REALLY puts me off buying WoD. Sure, I like other games that do not have flying, such as Rift, but I believe that is because the layout of zones/levels and levelling/endgame experience in the open world is much better. There are things to do inbetween each "daily hub" on Rift which gives it a reason to do these but WoW has not done anything like this in a long time. Is it because of flying mounts? I don't believe so. I believe it to be that the designers are not talented enough or/and limited resources to prevent them from making an engaging open world. The only way you are going to have a populated world again is to give people a reason to populate it, not limit their travel.
My friends will be buying WoD, I will watch them play and if the layout/quest design of zones does not support no flying to the way I like it, I will not be buying it. Why? Because I do not want to spend money on something I do not like, not because "will un-sub so they will revert a change so they can get my sub back cause I am a special snow flake". And also not buying because they said they will use the timeless isle concept again, fucking hate that zone with a burning passion, a strong example of poor zone design to me. Having to climb a mountain/hill shouldn't be a "path" it should be an opportunity to explore.
EDIT: I just used Rift as a reference, other games do it well as well.