The Rambam in his commentary to the Mishna list the 10 Nisayon's, challenges, which Avraham went through and passed. The first one is when G‑d tells him to leave his homeland to be a stranger in the land of Canaan. However, the second Pasuk in Parshas Lech Lecha writes: וְאֶעֶשְׂךָ לְגוֹי גָּדוֹל וַאֲבָרֶכְךָ וַאֲגַדְּלָה שְׁמֶךָ וֶהְיֵה בְּרָכָה. Therefore, with Avraham knowing what awaited him, why is this a challenge? Do any of the classical commentators deal with this question?
The baal (author of) Haflaa, in his book Panim Yafos, asked your question, and answered as follows: 12:4 says "וַיֵּלֶךְ אַבְרָם כַּאֲשֶׁר דִּבֶּר אֵלָיו ה׳ / Avram went as God had told him", meaning that he went not for the benefit promised him but merely in order to fulfill God's command. This, the baal Haflaa says, was his test: whether, once he knew the benefit he would gain from his trip, he would go for that reason or to fulfill God's will. And he did the latter, passing the test.
The test was in terms of commitment - though he was aware of the reward that would await him, he was not planning on leaving his father behind (who was presumably sick, and therefore stopped his own journey in Charan) [see Rashi at the end of Noach that Terach was still alive when Avram was told to go]. Also, it is clear that he would have to live forever in this new land, one that he was unfamiliar with, as well as deal with many outstanding questions: what would be his new source of income, could he purchase a home, would he find a place to live conducive to his spiritual path, etc.