It has been Allaah the Most High's way that whenever He sends a prophet, He gives that prophet certain miracles and signs to prove his prophethood to his people (for example, the miracles of Moses and Jesus).
In the case of the Prophet Muhammad (peace and blessings of Allaah be upon him), he was given the Qur'an as the ultimate proof of his prophethood. Note, however, that it wasn't the only miracle of the Prophet (peace and blessings of Allaah be upon him), but it is the only one which is permanent i.e. not restricted to a particular time or place. It is a miracle for all generations after the Prophet (peace and blessings of Allaah be upon him) to see and realise.
We can therefore appreciate that there must be something about the Qur'an that makes it stand out from all other scriptures proclaimed to be divine, something that indicates it is the truth, something that proves it is indeed the very Words of Allaah (swt).
The beauty of the Qur'an is that it is not miraculous in only one facet, but rather from all facets and angles that one can look at it. Thus the miraculous nature of the Qur'an cannot be limited to one list. In this thread it seems we have attempted to begin examining two or three of these aspects. However, many more exist, and it is important to consider all of the miraculous facets in combination, as the miraculous nature of the Qur'an is not by any one of them only. Examples of these are the following:
1. The language and style of the Qur'an
2. The incapability to produce anything similar to it by the disbelievers during the Prophet's (peace and blessings of Allaah be upon him) time and those after them.
3. The stories and accounts of the nations and prophets of old, since the Prophet (peace and blessings of Allaah be upon him) had no recourse to such information.
4. The predictions which occurred in the Qur'an, and which later came true
5. The perfect belief of Monotheism - the attribution of all that befits Allaah and the negation of all that does not befit Him, and the call of the Creator to the created to worship Him. All of this is not possible for a human to bring forth unless he was inspired by Allaah.
6. The laws and sharee'ah that the Qur'an came with, and the morals and conduct that it called for. All of this leads to the betterment of life in this world and in the Hereafter. The perfection of a set of laws that can be applied to any society at any time and place is humanly impossible, and the sharee'ah is the only example of such a set of laws.
7. Scientific facts mentioned in the Qur'an that were unknown at that time.
8. The fact that it has been protected and remained unchanged over such a long period of time, despite the fact that all other religious books have been distorted.
9. The compilation of the Qur'an and the diligence with which all the knowledge essential for its understanding has been preserved (such as the causes of revelation behind specific verses, verses revealed in Makkah/Madeenah, abrogation etc.)
10. The miraculous nature of the various ways and manners of reciting the Qur'an (the ahruf and qira'aat).
11. The ease by which it is memorised, and this is known by experience and observance.This is in contrast to all other religious books, for none of them are memorised like the Qur'an.
12. The deep meanings that are present inside it, and the fact that a reader never tires of reading the Qur'an, no matter how many times he has heard it or read it. This is in contrast to any other book, since a person cannot read it more than a few times without it getting monotonous and mundane.
13. The impact that the Qur'an has on those who hear it and the euphonious quality of the Qur'an.
Taken from An Introduction to the Sciences of the Qur'an, Yasir Qadhi, with slight modifications.
Each of these can be discussed in much greater detail, and the level of depth would be limited by the knowledge and research of those discussing it. But I hope it is apparent from looking at this list that the subject of the miraculous nature of the Qur'an is one that is very vast and that the previous pages of this thread have done little justice in that regard. Yet the sincere person who researches this issue adequately will soon find ample proof that the Qur'an is the very Words of Allaah (swt).
Regards.