al-Salāmu ‘alaykum wa rahmatullāh,
Arabic philologists have long studied the Arabic language in relation to the other 'Semitic' languages in an attempt to show the areas where Arabic is unique to the exclusion of the other Semitic languages.
The reaction of many Muslims who embark on Arabic language study and who have no prior experience of the Semitic languages is often to make ambitious claims that Arabic is unique in it's root system, or that 'it is the only language in the world in which you can begin a sentence with a verb.' Such claims are simply not true: the Semitic languages (of which Arabic, Amharic, Hebrew and Syriac are still spoken today) share a common characteristic of being based on a root system; and languages such as Welsh, Niuean, Malagasy, Gaelic, Hawaiian, Fijan, Tagalog, Maori and others – while they may not rank among the most widely spoken of the world's languages, nevertheless entertain a verb-initial grammar.
Extensive comparative study must be done of all the world's languages before one can justifiably lay claim to the uniqueness of Arabic among them. However, Arabic does enjoy certain characteristics unique to it among the Semitic languages, the most important of which are [*]:
It has preserved the sounds of the Semitic alphabet better than any of the other Semitic languages; it includes all the phonemes of the other Semitic languages (to the exclusion of perhaps two or three, about which there is doubt whether they are actually Semitic in origin), and also has extra phonemes that do not exist in the others, such as the thaa' ث, the dhaal ذ, the ghayn غ, and the daad ض.
It has the most extensive and precise grammatical and morphological system; all the grammatical rules in the other Semitic languages have their equivalent in Arabic, while there are many rules and phenomena in Arabic that do not exist at the same level, if at all, in the other languages.
It has more roots and lexical items than any of the other languages.
[*] Adapted from Fiqh al-Lughah by Dr. 'Ali 'Abd al-Wahid Wafi