To get an idea of the context of the time, we should listen to the music that came just before. Here is the Concerto in B flat, op. 13, no. 4 by J. C. Bach:
The Baroque concerto form consisted of a solo instrument (usually) set against an orchestra, the individual against the mass. Typically they are in ritornello form where the orchestra states a theme which recurs. In between the solo instrument has freer episodes. The big contrasts are not so much the actual thematic material, but the simple contrast between solo and group. The early classical concerto of J. C. Bach shares some of this repetitive quality. As you can hear in the concerto above, when the solo piano enters it is with the same theme that the orchestra had.
It was Mozart who made the concerto far more dramatic by changing the relationship between the orchestra and soloist. Instead of just repeating what we have already heard with a different tone-color (something rather lacking in drama) the soloist can have new and different material. Listen to this opening of one of his early concertos, K. 271 in E flat major:
Here the piano interrupts the orchestra after just a couple of measures to complete each half of the orchestra's phrase. And the drama is intensified by dove-tailing the first half of the phrase with the second half so we have a rather hasty seven-measure period. When the piano does return, not until measure 56, it is with a long, long dramatic trill.
In almost every other area, the symphony, the string quartet, Joseph Haydn was the forerunner, providing Mozart with fully-developed models of the form. But it was Mozart who was the real innovator when it came to the concerto. There are a couple of invaluable concertos, such as the one written for an early experimental chromatic trumpet, but in general Haydn's concertos are completely overshadowed by Mozart's. Here is the first movement of a Concerto in D major:
As you can hear, Haydn is making the same mistake as J. C. Bach: nice long orchestral theme, but when the piano enters, it simply repeats the same material. It was Mozart that saw what was wrong with this: the soloist must be set off dramatically from the orchestra, not just by being a solo instrument with a different timbre. He found many different solutions to the problem which I will explore in future posts. For now, let's listen to the first of his highly successful Viennese concertos, the Concerto No. 11 in F major, K. 413. Notice how the piano, when it enters, has quite a different version of the theme from the orchestra: