The game is actually built around gliding; scalable mountains are everywhere so you're not really short of a place to take off. So yes, it affects gameplay (and is actually gameplay in itself when it comes to raiding other players and getting the drop). You are able to glide over/to just about anything you want.
What you're trying to do here is point at other games and say "they didn't include flying, it *must* be because it's bad for gameplay" while ignoring every other facet of those games, not to mention the cost of developing the world with flight in mind in the first place. WoW can afford it. Most new MMOs can barely afford their promises as is. It's not a cheap thing to create.