Valithria Dreamwalker has really driven home a point for me: I am not fond of having to maneuver in 3 dimensions.
Oculus, Malygos P3, and Valithria have all had portions where you could move in all three dimensions. And I haven't really enjoyed those portions. It's not the vehicle aspect either. I like Flame Leviathian, and I don't mind jousting. Plus, in Dreamwalker you fly with your normal character.
Part of it is because I don't think I am very good at 3D movement. I always seem to move too far up or down unintentionally.
3D fights add a extra level of complexity that I am not sure makes an encounter more fun. For example, Malygos might have been better if the dragons were restricted to the plane. If they couldn't really move up or down. Would Valithria be any different if the orbs were on the ground, and maybe had to be killed instead of flown through?
But on the other hand, riding a dragon is all about flying in three dimensions. Maybe limiting it to two would be overly restrictive.