From a level designer perspective, the perfect level is a level that plays well without any visual cue whatsoever, the players get attracted to the objective and just look at what they have to look unconsciously.
You have to understand that textures/lights/props/overlays and such are small 'tools' that can be helpfull to guide the players in a level, they can help you to fix an area where you can't improve the design anymore. If you add textures/props and overlay too fast, you just 'hide' the design problem of your map.
Let me explain that further with an analogy.
I really like the 'Lean manufacturing' concept that was introduced by Toyota, I use it in my company to reduce my wastage and to improve my store by reducing the supplies.
One of the main theory explain that the stock is hidding the inefficiencies, and as a metaphor, they take a lake full of rocks filled with water.
The depth of the water may represent the size (the amout of stock): when the water is high many rocks are hidden and these rocks represent weaknesses.
If we are able to keep the depth of the water low then we can see the rocks: the weaknesses are visible and we can start approaching and fixing them in the most appropriate way.
You can use this metaphor in level-design aswell, if you put props/textures/overlays and such in your first alphas, you will add more water in your lake, this water is going to hide the inherent problem of the design of your level.
A perfect level is almost fullbright, contains two textures but still plays perfectly.