On the Jewish holiday of Succos, there is a ritual to hold four plant species and say a blessing.
One popular commentary:
- The citron (esrog) has taste and smell.
- The date palm (lulav) has taste and no smell.
- The myrtle has smell and no taste.
- The willow has no taste and no smell.
- Taste represents Torah knowledge and smell represents good deeds, so each of these four species represents a different type of person.
- When we bring together all four species, we symbolically show the importance of creating a community with all Jews and/or all people.
While this explanation is true, it leaves some unanswered questions:
- What is good about the willow? 
- What is bad about the citron? Or: if citrons are so good, why do we need the other types?
- Have you ever seen a community that welcomes people with no knowledge and no good deeds? What would such a community even look like? 
- This smell/taste commentary would seem to indicate a ranking between the four species, that some are better than others. But we don't see this type of ranking in the Biblical text, or in any of the customs around the four species.
- Why not use an orange? Why were these four plants chosen specifically?
After puzzling over this for a few decades, I added another question:
6. In the Bible, these species are listed in the following order: citron, date palm, myrtle, willow. Why are they in this order?
A pattern of four in the Torah
Over time I have come to believe there is a recurring pattern of four which shows up in many places, and this ritual is one example of that pattern.
I'm not able to find a source online, but I've heard one interpretation that says the Four Species are representative of Biblical Abraham, Isaac, Jacob, and Joseph.
I've read repeatedly (and don't know the source) that in Kabbalah these people correspond with chesed, gevurah, tiferet,and yesod respectively.
There is also a well-established tradition that the seven days of creation in the book of Genesis match up with the seven sefirot of Kabbalah.
To summarize, we have:
- Citron - Date palm - Myrtle - Willow
- Abraham - Isaac - Jacob - Joseph
- Chesed - Gevurah - Tiferet - Yesod
- First Day - Second Day - Third Day - Sixth Day
Using the above correspondence:
The citron, which has taste and smell, corresponds to the first day of creation, which was the creation of light. Rashi (Gen. 1:4) says that G-d created light on the first day, and then set it aside for the righteous in the world to come. We see a theme here of something that is both "good" and separate.
The date palm, which has taste and no smell, represents the person who has Torah and no good deeds. A date palm is fairly rigid, and in some traditions it binds together most of the other species. This is the person who does not compromise their principles to make other people happy. There is an opinion that the angels were created on the second day of creation.  I was taught that angels are like robots: they do G-d's will but they can't do good deeds on their own initiative.
The myrtle, which has smell and no taste, represents the person who has good deeds and no Torah. The third day of creation includes dry land and plant life. This is the first day where something is created which can reproduce itself: trees bear fruit which create more trees. Similarly, the myrtle can be compared to a rope:
There are many “thick-leafed trees” in whose branches “the leaves completely cover the stem”; but the Hebrew word avot (“thick”) also means “plaited” and “rope-like.” Hence the “branch of the thick-leafed tree” (anaf eitz avot) is identified as the myrtle twig, whose overlapping leaves grow in knots of three, giving it the appearance of a plaited rope.
Here is a verse from Ecclesiastes which ties these themes together:
"... a threefold cord is not quickly broken."
Finally, the willow has no taste and no smell, symbolizing the person with neither Torah nor good deeds. This is the sixth day of creation, which includes humans and land animals. As many philosophers have said in many times and places, humans can sink lower than the animals or rise higher than the angels. One meaning of "neither Torah nor good deeds" is that humans have tremendous potential, and no objective external constraints to force them into goodness or holiness.
G-d calls most of the days of creation "good", but G-d only calls the sixth day "very good":
"In each of the previous occasions in Genesis, when [G-d] saw that it was good, 'it' refers specifically to the next stage in Creation. This last one, however, refers to 'everything that [G-d] had made'.
A first layer of understanding
We can now start to answer some of the questions we asked earlier.
Q: What's bad about the citron?
A: The citron by default is separate, and can only fulfill its purpose as part of a larger community.
Q: What's good about the willow?
A: The willow has a lot of potential, as it can draw from both its own strengths and those of the larger community.
There are many customs of how to bind and hold the four species. My custom is to hold the date palm in my right hand and the citron in my left, with the willow in between the citron and the date palm. This particular arrangement leads to:
The role of the willow is to create the conditions and environment where the citron can join the community freely, without being bound rigidly by the bonds of the date palm.
Translating this back to the days of the creation and adapting Rashi's comment from earlier:
The divine light from the first day of creation is ordinarily hidden from view. We who were created on the sixth day of creation have a unique ability to bring this light into the world and make it visible to all creation.
Unity, diversity, harmony, synergy
The holiday of Succos has some universalist overtones:
- Sitting in the succah is the only Jewish ritual that involves the whole body.
- The 70 bulls sacrificed over the course of the holiday represent the 70 nations of the world.
- The rules for constructing a succah say that it could potentially be so large that all people could eat in it.
The Four Species have universalist overtones as well. At the cosmic level, we have the ability to unify all the forces of creation. At the communal level, we can create synergistic communities that do not exclude anyone. We can have unity within: intellect, spirit, emotions, and body. Finally, the easiest and hardest is to bring our full selves into this ancient ritual, with so much depth and richness and meaning.
 Many have already answered this question, for example: https://www.chabad.org/library/article_cdo/aid/4515/jewish/The-Unity-of-Our-People.htm
 Robert Fulghum expresses the same sentiment beautifully in his short story about mermaids, in "All I Really Need to Know I Learned in Kindergarten."