Useful rule

The demo they showed off was more about using Portals assets as something familiar for the fans rather than recreating the regular portal experience, hence why it was just a player standing in a single room interacting with largely stationary objects.

Trying to play regular Portal in VR would be a disaster due to the extreme motion sickness it would cause in anybody who tried to play it for long enough due to the games nature of propelling the player around maps and disorienting them. Even relatively tame FPS titles as far as movement are concerned can lead to extreme motion sickness in players using VR because they typically lack any sort of anchor for the players centre of balance to "hold onto".

Your physical body knows that you are not moving because you are sitting in a chair but your eyes are telling your brain that you are moving and this inconsistency leads to nausea, vomiting, sweating, instability, headaches etc.

Racing games and flight sims can combat this by showing you a cockpit/chassis that aids your brain in understanding that you are not moving relative to the cockpit. ie. you might be flying between buidlings in your jet but your cockpit remains relatively stationary around you and this stops your brain from freaking out and making you feel sick.

FPS games meanwhile lack these sorts of features and for VR to work with them fully they will need to take steps to combat this, either by including some sort of cockpit view (think seeing the cockpit of your titan while inside one in titanfall) or by advancing VR sims so that you are actually physically moving (albeit this will be way harder both in terms of practicality, time and expense).