I think back to original flying mounts in WoW; the normal mount was actually slower than an epic ground mount. The only advantage was being able to avoid enemies and terrain, but it wasn't actually faster than hopping a horse. The mistake lay in the epic fliers, who were ALWAYS the best choice. If they made it so ground mounts were always faster than winging it, you'd actually have a choice to make. Blade's Edge? Fly. Nagrand? Groundmount. Add in some AA batteries with faulty IFF transponders to make hazardous or outright mo-fly zones and you've got some interesting ideas. Add a couple Sidewinder A2A missile racks to mounts and we're getting into fun times.