Va'avarechah mevarachecha umekalelecha a'or - I will bless those who bless you, and he who curses you, I will curse
This implies that the people of Ur Kasdim were cursing Avraham and that had prompted G-d to tell him to move to a land He would show him. There he would become a source of blessing instead of the recipient of curses. G-d would henceforth curse those who cursed Avraham, be they individuals or groups of people. The Torah did not want to elaborate on the causes why Avraham was so disliked, just as it had not spent many words on describing the religious arguments which went on during the time of Enosh. (RaMBaN)
There is another approach to the meaning of the words, "and he who curses you, I will curse." It may refer to King Nimrod. The Torah did not need to write the word umekalelecha in the plural as once G-d curses the King of a nation this means that the entire nation will suffer the curse. It appears likely that the people of Ur Kasdim engaged in a variety of forms of idolatry, different sections of the populace believing in and worshiping different deities. Some people served the Molech, believing that if they sacrificed one of their children to that god, he in turn would grant them success in the rest of their endeavors. The Molech's priests promised that his adherents would become great nations. Other people, interested less in political success but in economic success, would worship another deity which symbolized such success. Still others believed that by worshiping the right deity they themselves might be appointed by that deity to become its prophet and to enjoy its powers on earth.
Considering all these factors, G-d promised Avraham that He would make him into a great nation, that He would bless him, that He would make him a great name, and that he himself would become a source of blessing, all in order to counter the various claims to the contrary which the different followers of different deities promised to their followers. These words were a message that all the claims made by the various priests of the various deities were devoid of content and that they had no power to influence their followers' fates either positively or negatively. None of the forces of nature considered as primary sources of power by those who worshiped them has the power to cause any changes in the universe except within the parameters set for them by their Creator. They most certainly could not act in a manner which is the opposite of their generally perceived function. It was extremely foolish to expect that a planet such as Mars which is perceived as a harbinger of war and bloodshed should become a source of blessing.
This is why G-d added, "I will bless those who bless you, and those who curse you I will curse," to underline that only G-d Himself is the source of both blessings and curses. The only power in the universe which is able to influence events in our lives in either direction is the Creator Himself Who has created the other forces to act as His agents. This is what the prophet meant when he said, "not like this is the portion of Yaakov, for it is He who formed all things" (Yirmeyahu 10:16). He mant that both good and evil emanate only from Him. This is also why Yeshayahu 45:7 said, "I Hashem do all these things."
The homiletical approach (Tanchuma Lech Lecha end of section 4) sees in the words va'avarechah mevarachecha (I will bless those who bless you), a reference to the Kohanim who extend G-d's blessing to the Yisraelim see that after the Torah commanded the Kohanim to bless the Yisraelim, the Torah concludes with the words, "let them place My Name upon the Children of Yisrael, and I shall bless them" (BaMidbar 6:27). This is equivalent to G-d saying, "I will bless this tribe." G-d said, "in this world the tribe of Levi will bless you, whereas in the Olam Habah (World to Come) I will bless you directly." This is also more directly alluded to in Yirmeyahu 31:23, "Hashem bless you, abode of righteousness O holy mountain."