Parts of the Body
Once you know this analogy, you can actually see it (at least I do). And if that doesn't make this weird ritual even weirder, I don't know what will.
Palm Branches: Spine
As you use the lulav and etrog to worship Gd, so do all of these parts of your body worship Gd.
The Four Types of Jews
Since unity is one of the major themes of Sukkot, we look at the lulav and etrog as a collection of the various types of Jews. However, I think this particular symbolism is more likely to make you disparage another Jew than bring unity.
Each "person" is classified according to his learning and his deeds. A nice taste symbolizes learning, and a nice fragrance symbolizes one's good deeds.
Etrog: Has taste and fragrance, so this represents a Jew who is learned and practices good deeds.
Palm Branches: Taste and no fragrance, which represents a Jew who is learned but does not do good deeds. (See, you just thought about who might fit that bill, didn't you? That's why I don't like this exercise. #BadMiddos)
Myrtle: No taste but fragrant, so this is the simple Jew who does good deeds.
Willow: No taste and no fragrance, so I guess that makes it the rasha of the group. But perhaps the one who is learned and does not do good deeds should be considered even more a rasha because he should know better?
I prefer to look at this analogy as being parts of ourselves. In certain mitzvot, you may be the etrog, and in others, you may be the myrtle. But on the other hand, maybe you're the willow for mitzvot you don't understand, and maybe you're the palm branches when you feel defiant of a mitzvah you know.
As a bonus, here is another symbolic meaning of the 4 species: they represent the agricultural abundance in Israel and Gd's role in creating it. Each items is tied to a particular habitat in Israel, and water is required for each. As we enter the winter, so begins the rainy season in Israel. In fact, this begins the time of year when we daven for rain in Israel during the Amidah 3 times a day. (Don't forget to add those parts!) This symbolism leads to people call shaking the lulav a "rain dance," and that's too pagan for my tastes.
Unified Field Theory
Yes, you read that right. We're going to dive into theoretical physics! (One of my favorite subjects!)
I don't trust myself to summarize this theory, so you should just read it here from Rabbi Avraham Arieh Trugman (it's very short, I promise). But here is the general idea:
"By shaking the four species outward to the six directions of space and then bringing them back to our hearts, we unify and sanctify space within time."
Does any of this symbolism resonate with you? Tell us about it!