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For ease of writing, assume that "skill" refers to some mix of time and skill.
That triangle is a rough representation of a theoretical population. It is a skill distribution, tipped vertically. In this form what it represents is the skill requirement that players will get stuck at. You might notice that it has a slight positive skew, that is because I am bad at drawing. This is supposed to be a bell curve, but it doesn't really matter.
The lines represent the entry barrier to raiding. Or more accurately, they represent a first boss kill, with the idea being, if players get past this, they are now active raiders with raid guilds, but they may fail to get past the second boss, or the last in the last raid at the highest end. This does not indicate how much content there is at each skill level, but it is easiest to visualize, and the most straightforward, if we view it as a chain of bosses of steadily increasing difficulty, which the players will then kill or fail to kill. To be clear, anything above a line is raiding. Anything below the line is not raiding. Incidentally this means that Line 4 shows a really, really hard non-raiding game, but that's not the subject of this post. Let's assume that anything below the line just doesn't exist (devs didn't make the content).
Line 1 means that the lowest raids are really easy. It also means that players will tend to cluster further up along the distribution. This makes it top-light and bottom-light and is a perfect setup for a "left behind" scenario. This is due to most players being able to get well past the entry content. In fact, it is so easy that only a small portion of the population is bad enough to not get past the entry content. New players may find that there are not enough player to play with.
Line 2 is a system in which there are a lot of players clustered near the bottom. If we pretend that the triangle means anything at all, there are 3-4 times as many players in the bottom content, which will reduce the prevalence of left behind syndrome. There are, however, more players excluded from the higher tiers. Take note of this: excluding more players from higher up content makes it easier for those who have the skill to advance.
Line 3 is an exclusive system in which most players are not getting past the entry tier.
Line 4 is a more extreme version of line 3. And it probably represents a game that barely anyone plays. Don't make a line 4 game.
Note that no matter what system you choose you will have a dwindling pyramid at the top. This means that top guilds will always have more recruitment difficulty than the rest since they are in content that most people cannot do. There are two ways to fix this. One is to get a bigger triangle: have a larger population. The other is to crush or chop off the top tier, causing the highest guilds to be doing content below their skill levels. This is inevitable in practice, since I doubt any dev team could create content of exactly the right skill level for that very very top of the pyramid (meaning almost, but not quite, impossible, since impossible isn't very hard to make). We see this in WoW, where top guilds aren't necessarily doing harder content, but the same content as players a few steps down, a lot faster.
This model has some limitations. Such as being an ugly drawing. But also that it models a perfectly linear progression system, which does not exist, since this assumes very exact rewards from previous bosses, such that killing boss A once means you have the gear for boss B, but maybe not the skill, and killing boss A again won't make boss B any easier. Despite that, I think this is a useful visual tool. If it wasn't so damn ugly.