The Midrash teaches that the four species represent four types of Jews, with the arava (willow) representing the lowest level, the person who lacks both Torah study and good deeds. The Midrash further compares the four species to four body parts. The arava, with its long, lip-shaped leaves, represents the mouth.
Is there a connection between the mouth and a person who lacks both Torah and good deeds?
When Hashem formed man from dust and breathed life into him, the Torah calls him "a living soul," which Targum Onkelos renders as "a speaking soul." Onkelos departs from the seemingly simple translation to teach us that speech is the essence of man's nature, that the mouth is the essence of humanity.
But what was the purpose of this wonderful gift? With no one else yet created, to whom was man going to speak? To the trees? To the birds?
Rather, the purpose of speech was for man to communicate with his Creator, to distinguish himself and raise himself above the rest of Creation.
The simplest Jew, lacking both Torah and good deeds, still retains "a mouth," for "Hashem is close to all who call upon Him." One need not know how to talk to Hashem; one need only appreciate that he is indeed speaking to Hashem.
Certainly the numerous laws and ever-deeper levels of understanding enhance tefillah (prayer). However, the essential factor is that the Jew speak to Hashem from the heart. In this, the "simple Jew" can sometimes exceed those involved in Torah and mitzvot, for he knows he has no impressive "gifts" for the King, so what he offers he offers with a full heart.
On Hoshana Rabba, we put aside the other three species and wave a bundle of aravot by themselves. On Hoshana Rabba, even the simplest Jew, the "arava Jew," now stands on his own feet. Although his lack of knowledge and practice makes him incapable of being deemed righteous, he is capable of flashes of inspiration and clarity that the truly righteous might rarely achieve. Recognizing his lacking and his distance from the Creator, he can take advantage of this distance through his longing to return. His tefillah can be innocent and ingenuous, imbued with tremendous inspiration. With his "back to the wall," he musters all his energy and cries out to Hashem. And in truth, every Jew, no matter how learned or righteous, is a simple Jew.
The province of every Jew, tefillah has throughout the ages remained the treasure of the oppressed and distraught, and of the simple, unpretentious Jew. Tefillah is the most basic gift of the Jew, to communicate with the Creator. It is open to every Jew who is willing to labor - the labor of the heart.