AltMuslimah carries an article by Sister Mariam Khan about why she wears an abaya:
"The decision to wear an abaya has, however, set off different alarms. It has actually been somewhat of a wake-up call. It's the realization that a woman can look simple yet elegant at the same time, discreet yet beautiful. Most importantly though, it's knowing that the choice to wear an abaya is solely to gain the pleasure of Allah, and that is what makes wearing it an act of worship. Hence, the abaya is a form of submission, not to our husbands nor to our parents, but to Allah alone.
After researching the issue and reflecting upon the evidences behind it, I learned the abaya is a source of freedom, rather than oppression. Freedom from having to keep up with annual trends and fashions. Freedom from the "shop-aholic" syndrome many women claim to suffer from. Freedom from standing in front of a closet filled with clothes, yet clueless as to what to wear. On the contrary, I experienced freedom of movement with the abaya. The freedom to walk about freely without attracting negative attention from men. The freedom of being a mere object of man’s desires."
You can read the full article here.