I used to think 'no icons' would be too unrealistic given the limitations of the monitor/graphics etc. After spending time in a non virtual cockpit, the experience of no tags is similar to the real thing.
Spotting aircraft in the sky is an Active, rarely passive, process. Planes don't jump out at you from the corner of your eye. If you are not actively searching you will often never see them. Pilots are trained to follow specific search patterns. Once spotted it takes a second or more to study the plane to discern heading, speed, closure, etc. It is common to have planes dissappear and reappear depending on the planes aspect to you and lighting conditions.
Another aspect of flying is that your eyes are either in or out of your cockpit. You can not follow an aircraft and look at your airspeed indicator at the same time. It may be a quick glance but you can't do both simultaneously.