G-d said to Abraham," ... Whatever Sarah tells you, heed her voice."
Hagar's son, Ishmael, was showing himself unreceptive to the moral education he was receiving from Abraham. In order to prevent him from being a negative influence on Isaac, Sarah insisted that Abraham banish Hagar and Ishmael. Abraham was reluctant to do so, but G-d validated Sarah's judgment.
The more attuned a prophet is to the affairs of this world, the greater his or her degree of prophecy. Since Abraham was somewhat detached from worldly matters, he could not perceive Ishmael's true negativity. Sarah, in contrast, was more involved in worldy matters, so she readily was able to perceive Ishmael's evil. Her prophetic vision was therefore superior to Abraham's.
Furthermore, we are taught that in the Messianic Era, the feminine aspect of creation will rise above the masculine. Abraham's and Sarah's spiritual refinement was so lofty that it enabled them to experience a fortaste of the Messianic Era. For this reason, too, Sarah's prophetic vision was superior to Abraham's.
Nowadays, as we approach that time, all of us can welcome the flowering of feminine power in the world, acknowledging that women's more intense experience of physical life grants them a higher level of spiritual insight then that given to men.
--Excerpt from Kehot's Daily Wisdom