You may still have to train users, but a little training can go a long way
3D interaction is often thought of as "natural," but for many novice users, effective operation of 3D UIs is anything but natural. Users in HMDs don't want to turn their heads, much less move their bodies. Moving a hand in two dimensions (parallel to a screen) is fine, but moving a hand towards or away from the screen doesn't come naturally. When using 3D travel techniques, users don't take advantage of the ability to fly, or to move sideways, or to walk through virtual walls (Bowman et al., 1999).
Because of this, we find that we often have to train our users before they become proficient at using even well designed 3D UIs. In most of the HCI community, the need for training or instruction is seen as a sign of bad design, but in the examples mentioned above, effective use requires users to go against their instincts and intuitions. If a minimal (one-minute) training session allows users to improve their performance significantly, we see that as both practical and positive.