But is it possible to pin down all the things that make Mozart Mozart?
Hehe! No. Mozart's style is "classical", often using a very correct form of 18th cenutry sonata form; also he is known for his especially clever and
subtle harmonic progressions, his specialty, a very 'natural feel' to them. To generalize, he is often melodic and lyrical, serene with brief interludes of conflict and darkness. His music is not "intellectual" but very clear, directed, and capable of subtle emotional inflection; at times very moving for no reason in particular. If you want to consider development, his major influence was probably Haydn, and in a general sense his music evolves over his lifetime to less directness and formality, more sublety and color. This just a short summary, other posters may be more thorough.