Your audience is good at recognizing problems and bad at solving them

Mark acts as spokesperson for the game. He's very kind to answer as many questions as he can and be open about what he can talk about to the players. Over the course of many years he has answered thousands and thousands of questions. The metaphor that he uses is about going to a doctor. The doctor knows the diseases and symptoms better than the patient and as such, he or she is better equipped to solve the problems that are found in the patient's body or mind. I'd make an addendum here. Depending on the doctor they might be incompetents that know less than the patient himself. And the opposite can also be true, an arrogant patient that pretends to know more than his or her doctor. But that's not the matter of the subject here.

The point is, the players of your game are well versed in it. They know the strategies. They know its virtues and defects. After all, if they truly love your game they also have a deep understanding of it. However, they have the point of view of a player, not a designer or developer. It's easy for them to notice a problem. Solving problems is another matter. The players don't know how the game was made, the process behind it. They don't know what restrictions were in play. They don't know what, how many and how decisions were taken. Another addendum. Let's assume that most players are just players, not developers or designers. Mark himself is talking about the general public.

Does that apply to level design? Absolutely. Most players are easily going to notice problems in level design. If the player has trouble navigating the level or finds it too hard to complete they'll know because they are experiencing it themselves. If the player gets lost they are going to criticize the level. If they find a level or parts of it annoying they'll feel annoyed. If the level is too long they are going to feel bored / tired. As a side note not all players are going to have the exact same experience, we are talking about an average here. The level designer must see the level from the perspective of a player, else they risk pleasing themselves and not the players. A level designer won't be able to examine their own creation from the perspective of every player, with thousands or millions of players this is an impossible task. The important thing to take note is that in the same way Mark is all ears to listen to the players, a level designer must also speak to his audience and be willing to listen to them.

A self-critic: this site has many critics about level design. But it's impossible to know the whole process and all the people that were behind the games. Sometimes a level isn't bad because it's bad. There may be other factors playing its role such as time constrains, exhaustion, technical difficulties and other unknown factors. That or the matter is beyond being good or bad, but related to intentions, philosophies or goals behind a level that I don't know.

As a bottom note I'd add one more thing that Mark didn't mention. We often confuse what is a personal taste with a mistake. Each person has its own tastes and ways of doing something. When we have food, clothes, arts and things that relate to emotions and perception. We have to remember that what we are comfortable with and what we understand are part of a reality that is very much personal. I'm not going to discuss philosophy or psychology here. The experience from player to player is going to be different because they are different people. The confusion happens when a player confuses their personal taste with a mistake. A player may like a certain type of skin, a certain character in the game, a certain type of move, etc. As Mark's other lessons teaches, players bound to something in the game and players bound to different things.

How to differentiate a personal taste from a mistake? In Magic a mistake happens when they make a mechanic that is not understood or not well received. (there are multiple types of mistakes but I'm focusing of these two for now) If the players aren't having fun or aren't understanding something, that's a good indicator of a mistake. Now think about who made a mechanic that wasn't well received by the players. We probably have there some idea that seemed great for whoever had it, but it wasn't great to the player's eyes. Note the plural, players. Not just one player, yourself. Let us shift to the player's perspective now. I may have fallen in love with a certain level, a certain character, some spell or weapon in some game. I don't know who made those and what the makers were thinking or feeling, but I've fallen in love nonetheless. Suppose that the things that I fell in love with are hated by most players of that game and most of them avoid those things. If the makers of that game were to poll the players, I'd be biased and more inclined to vote for them to keep those things, while the other players would vote for those things to be replaced or removed.

We have a matter of ego in the previously hypothetical scenario. What is more important? My personal taste or the game's environment? From the perspective of a game developer, what is more important, your personal ideas or the cohesion and coherence of the game itself? In Magic they have the tool of banning cards when they think / feel (I really can't say for sure) one or more cards are warping the environment. There is one dominant strategy that is suffocating others and this is unhealthy. If I'm a player and the game has 20 levels, with two of them being played way more often than the others. I've learned at high school in geography classes (God bless that teacher!) that when people move from one city, state or country to another we have to take into account two main driving forces: one force is pushing the person away from their origin; the other force is pulling the person towards their destination. In my example there is a force pushing players away from 18 maps, while the other two maps have some force pulling the players towards them. Players are usually more biased in seeing a problem in a game because they value their tastes first, their personal experience. On the other hand, the developers of the game have to please a broad audience. The developers have other priorities and challenges of their own that they players don't have to solve or don't know about.