Being conversant with vocabulary and having a good knowledge of words at one's fingertips is not such a great difficulty, but their combination and arrangement and harmonisation in a style which has regard for eloquence and fluency, and the construction of phrases with a variety of expression yet in the same way in which they arise in the mind, is the most important skill, and it is something which is not practicable without observing the narrow rules of literary exactness, and using craftsmanship and eloquence. In the art of eloquence, it is said that for eloquence in any speaking or writing it is necessary to observe three principles.
1) Proficiency in words and their meanings.
2) Power of thought and subtlety of eloquence.
3) Power of expression, or skill with the pen.
But it must always be kept in mind that although all the rules and requirements of eloquence may be heeded, no one can claim that his speech or writing is always the best, and that no one can parallel him. However, Allah the Exalted, Whose range of power and knowledge is without limit, has so variously decorated His words in the Qur'an with arrangements and harmonisations of words that no one, be he the most eloquent man on earth, can bring its like. And this is the secret of the eternity of the Qur'an, the |everlasting prophetic document of Hazrat Muhammad (S.A.). The Qur'an, according to the testament of history, shone at that time when the Arabs were at the zenith of their literary development.
The famous poets and great orators, Imra'ul-Qais, Labid, etc., who are still counted as outstanding geniuses in the field of literature, wrote poems and gave orisons which sometimes reached the limits of greatness and which were written on curtains and golden plaques and attached to the wall of the Ka'abah. But, with the rising of the brilliant sun of the Qur'an, all of these lost their light and were eclipsed like the stars. The eloquent Arabs were left bewildered by the eloquence of the Qur'an, which was such that the enemies who were full of hate for Islam and Muhammad (S.A.), who even took to the sword to wipe him and his religion out, were unable, with all their efforts, to find even one short mistake in the language and expression of the Qur'an.
source : http://www.maaref-foundation.com