In a 'nutshell', our modern English is a literary creole of middle age W. Germanic (e.g., Anglo-Saxon) and N. Germanic (e.g., Norse, Danish), relexicalized on Norman French (and remember the Norman conquerors were themselves Frenchified Danes), Latin and Greek.
The phonology is thoroughly and demonstrably Germanic (e.g., a large number of vowel sounds and vowel combinations), while the spelling is thoroughly Latin/Romance/French. In addition to the common vocabulary, one way in which written English and written French are much alike is that the spelling systems preserve etymological relationships at the price of phonetic clarity.
English's uniqueness comes about from (although you can show other major languages undergoing similar phenomena through linguistic and cross-cultural contact) the following:
1. its earlier forms losing their W. Germanic and N. Germanic inflections,
2. its largely foreign lexicon,
3. the mismatch of a phonology more akin to Dutch or something like that with a French spelling,
4. a creole grammar of mood, modality, tense and aspect dependent on word order and word combinations,
5. and a polyglot system of lexical derivation.
It's 'greatness' as a language comes about from the hegemony of first the British Empire and then the American post-war Neo-Imperium, along with the reality that Anglophone countries like Australia and Canada, although they have relatively small populations, have large economies and are well integrated into the sphere of US hegemony.
University of Fukui, Japan