The comment on the last post has me in a writing mood.
In Mozart's day, the gist of a symphony was 4 movements: each one would basically have 2 musical ideas which were presented separately, then interplayed in a middle, 'development' section.
Mozart was very clever and would sometimes introduce 3 ideas. Sometimes, when 200 years of convention would dictate that an idea should be in a major key (given the previous idea), he would use a minor key. Subtle, but witty stuff.
Beethoven kicked down the door, ushering in the Romantic era. Though not the first example, the best example is the ubiquitous 5th symphony. It takes that single idea (you know it) and pursues it furiously -- almost insanely. It was terrifying for audiences of the day.
Beethoven broke all kinds of rules. The glorious 9th symphony re-uses ideas from other movements (crazy!) and of course employs a chorus in the finale, Ode to Joy (influencing a famous tech blogger some 200 years later).
ps. Their personal lives rivaled any rock star of today. Esp. Liszt and Paganinni.