Social factors
The economics of music changed, too. Composers wrote for patrons and used local musicians known to the families concerned. Parts for this type of music making had to become simpler - with occasional pieces for virtuosi on particular instruments. Music could be copied and circulated more easily throughout Europe, feeding the growing cult of composer celebrities.
Notation
Melody lines were notated in increasing detail with phrasing and dynamics. Gradually the signs used became standardised, although manuscripts from individual composers are still recognisable to experts.
Movements were unified with distinctive moods and rhythms or tempo.
The Baroque habit of making each movement devoted to a single "affect" or emotion faded away. Form as we know it was born - with contrasts between sections, managed with key changes, stridently rhythmic themes next to lyrical ones...
Try to play
a movement from a sonata by one of these great classical composers for the piano: 
