Two significant attributes of agency are:
1) There is no agency for a transgression; the forbidden act on the part of the agent is not attributed to the principal (SA CM 182:1 in Rema);
2) a non-Jew can generally not be made an agent even for commercial transactions (CM 188:1).
It is obvious that a person can have another person perform a physical action on his behalf, like hiring a worker to plow a field or paint a house. But it is a bit surprising that someone can actually delegate his unique personal ability to make decisions and to effect legal agreements to someone else. After all, if we are taking about a mitzvah, we know that these require the specific intention (kavanah) of the mitzvah-doer (SA OC 60:4); if we are talking about a commercial trans- actions, the validity of these trans- actions is based on a "meeting of the minds" How can the principal (the sender) provide intention or agreement when he is not even present? And why is this ability lacking if the agent is a non-Jew?
Rebbe Natan of Breslav has a unique and compelling explanation for this ability. He explains that ultimately, even the power of the individual to make his own decisions is really a kind of agency from the Creator. After all, we are not placed here on earth to pursue our own vain desires, but rather to carry out HaShem's will. Of course He is capable of carrying out whatever He desires, but He chose to create human beings in His own image and to give us free will, at the same time instructing us to use this will an an extension and fulfillment of the Divine will. So all of our acts of will are meant to be in the service or agency of G-d.
In the plan of Divine Providence, each of us has our own unique mission which we need to carry out using our will and our judgment. So it is still true that "the mitzvah is greater when done by him than when done by his agent". But ultimately, this mission is that of carrying out G-d's plan, which all members of His people are equally charged with fulfilling. So a person's agent is indeed, like himself, a representative of the Creator who is authorized and charged by Him (Likutei Halakhot Breslav, laws of agency 2).
This approach explains the remarkable prevalence of agency regarding the per- formance of mitzvot. The acts that most prominently express our adherence to HaShem's will are of course are fulfillment of the commandments of His Torah. This approach also makes it obvious why there is no agency regarding a transgression: even the principal (the sender) would not be fulfilling the will of G-d by carrying out a transgression; it follows that the agent lacks the power to act on behalf of the principal in transgressing, since there is no longer a chain which extends from G-d's will, which is the ultimate source of the power of agency. (Indeed, there is an opinion in the gemara which states that any act done in transgression of the Torah lacks legal force, even if not performed by an agent (Temurah 4b).
We can also understand why non-Jews are not valid as agents; they are not commanded in the mitzvot of the Torah and therefore are not completely "deputized" to carry out His will in all of its expressions. (Of course all human beings are created in G-d's image and are obligated to use their free will to fulfill His will; the difference is in the extent of the authority and obligation.)
A most remarkable aspect of this explanation is that the effectiveness of agency in commercial transactions is attributed to this same mechanism.
Indeed, this essay is found in the "Choshen Mishpat" section of Rebbe Natan's book. Our commercial dealings are not a "side-light", a distraction, from our service of G-d. On the contrary, these dealings are among the most important ways in which we act as agents of the Creator! In these dealings also we are only able to fully delegate our power of will and judgment to others who are equally commanded to per- form of Torah's commandments.