I played this once...

This piece (1st movement of Mozart String Quartet K. 421) is, in my opinion, very cool. I loved playing it and it's interesting to analyze it now that I've had that experience I suppose.

The exposition is divided as follows:
exposition: measures 1-41
first theme: measures 1-8 (with two phrases, each 4 measures long)
transition: measures 9-14.2 (made up of two phrases, one from measure 9-10 and the other new phrase includes material from measure 9 but possesses an extension up to measure 14. 2)
second theme: measures 14.3-41 and the internal phrase structure of the second theme is as follows--

mms. 15-18 phrase
mms. 19-24 phrase
mms. 25-26 small phrase
mms. 27-28 small phrase
mms. 29-33 variations of the small phrase heard in mms. 27-28
mms. 33-41 an extended coda (could be called closing material) that builds in each voice (inverted counterpoint!)

Because this movement is in D minor, the expected key area for the second theme would be III, however, this does not occur. It modulates to the key of C major with a big C major chord on the third beat of measure 14, the beginning of the second key area.
The second theme and the first theme contrast in different ways than key areas, however. The character of each section is different. At the beginning, the first violin and viola parts are marked "sotto voce" which means half voice. This, in my opinion, is a mournful sounding section of the piece and that should be considered when performing this. Performers should consider what this means for the performance, including bow strokes and contact points of the bow on the string. However, the second theme presents itself as completely different. It begins with a large forte chord and a sudden drop to piano. Perhaps this is meant to be startling--the abrupt change to piano signals a previously absent playfulness in the music that should be emphasized through light bow strokes and some short, staccato eighth notes. The second theme is also developed much more than the first theme--it is developed through the tension of inverted counterpoint in each part that slowly builds to climaxes that are then surprisingly followed by playful piano sections once again. Performers should certainly pay attention to the dynamics and articulations because they do much to color the second them as opposed to the somber first section.
I believe that the codetta could certainly be labeled as closing material. First of all, it is much longer than a "typical" coda of only 2-4 measures. It encompasses 9 measures and has its own phrase structure that includes the building of tension through inverted counterpoint and repeatedly transposed motivic material throughout.
Typical string quartet structures are as follows:

1. one instrument plays a solo part while others accompany: mms. 1-9; mms. 25-32
2. the first violin and celo play a duet, accompaniment in viola and 2nd violin: I actually couldn't find any of these places in the exposition...
3. a motive is passed through the quartet: mms. 12-13.2; mms. 20-23; mms. 33-41

The internal structure of the recapitulation is as follows:

recapitulation: mms. 70-end
first theme: mms.70-77 (with a phrase from mms. 70-73 and another phrase from 74-77)
transition: mms. 78-83 (with a phrase from 78-79 and another phrase from 80-83)
second theme: mms. 83-102.2 (with a phrase from 83-88, extended material developed through each instrument from 89-93, a phrase in the first violin with accompaniment from the other parts from 94-102, and starting at 98 there are variations from the first violin of the previously heard theme)
closing material/codetta: mms. 102.3-117(end) (more extensions of previously stated material)

The entire recap is in the tonic key, and this is made possible by the chord present at the beginning of the second theme. Because of this, the recap differs from the exposition. The rhythm of the second theme in the recap also differs from the rhythm in the exposition. There are longer syncopated note values present in measures 17-18 of the exposition and there is a rhythmic diminution of these notes in measures 86 and 87 of the recap to make up for lost time. This reveals that metrical practices in the Classical era were extremely calculated and well thought-out. This clearly reflects enlightenment thinking of the era as well.
The end of the recap facilitates repetition because it does not necessarily signal the end of the piece. There are repetitive motivic figures in the first violin as well as the second violin and the viola. These figures do not seem like they would lead into the end of the piece, but that they would introduce new or repeated material, which is precisely what they do.
The development begins in the key of Eb major with a large rolled chord in the first violin. There are three motives that are developed throughout the development, beginning with motive "a" at measure 43, motive "b" at measure 50, and motive "c" at measure 59. Motive "a" is passed throughout the parts and is a trilled dotted eighth note-sixteenth pattern followed by two eighth notes and a half note. Motive "b" is played by all parts at the same time (though all parts have different notes) and is a descending type of scale pattern. Motive "c" is a progression of five sixteenth notes followed by an eighth note.
The retransition occurs at measure 53. Here, all parts drop out except for the viola which has a solo line that eventually is built upon by the other instruments. This signals the retransition because it is new thematic material and eventually builds up a lot of tension before releasing it all at the appearance of the recapitulation at measure 70.
The scoring of this section is as follows:

1. one instrument plays a solo while the others accompany: mms.42-45
2. the first violin and cello play a duet while the others accompany: mms.63-65
3. a motive is passed through the quartet with each instrument playing it in turn: mms. 53-62
4. the first and second violins play a duet: mms. 46-49

This piece is wonderfully written and is so much fun to play! Too bad it's not as fun to analyze...:(