As with so much sublime Mozart, it's hard to put into words (warning: feeble attempt ahead) what makes this so satisfying, although you can hear and see for yourself here. (The solo parts begin at about 2:14; note that in this case Fischer and Nikolic have been playing along with the orchestra from the beginning.) The first thing that struck me in thinking about this opening is that pedal tones are prominent; I've written several times in the past that I'm putty in the hands of a perfect pedal point passage. In a sense, the way a pedal point passage functions is a sort of stripped-down, elemental picture of how Tonality works. In tonal music, there is almost always the sense that a certain pitch acts as a center of gravity for our hearing - with a pedal point, we just get to hear the center explicitly. (Except when it's not explicit. In Mystislav Rostropovich's inspiring discussion of a dominant pedal in Bach's 3rd cello suite, he makes the point that he hears the pedal pitch sustaining for much longer than just the measures where it's actually sounded - the power of suggestion is so strong that, until that dominant is resolved, we hear it without hearing it.)
The entire 71-bar intro to the Sinfonia Concertante functions essentially like a tonic pedal, even though only about 40 of those measures have a sustained E-flat - the bars where E-flat is not omnipresent are generally short cadential patterns that are also reinforcing the tonic, so it's basically two minutes of building tension over an E-flat. (Curiously, perhaps the most famous of all pedal points is the sustained E-flat with which Wagner's Ring commences; however, that music could hardly be more different than this.) It might seem odd to say that tension is building since E-flat is the home pitch, but Mozart manages to achieve the sense of expectation in several ways, not least in the sfp's of the two opening bars. One of the many features I like about the Fischer/Nikolic performance is the way these opening bars are handled; in some respects it's a very formulaic fanfare opening, but it's also a good example of how much articulation can matter. First a full-measure tonic chord is struck loudly and immediately turns soft, followed by a half-measure repeat of this gesture, followed finally by a half-measure dotted figure which seems to explode from the sublimated energy of the first two chords.
[Click score excerpt to listen.]
Now we're on our way, but the tension really builds from 1:29 to 1:49 with a series of trills rising in a chromatic pattern over the same E-flat. (Nikolic looks like a jack-in-the-box about to pop during this section, and he clearly loves playing the climactic viola section trills that follow.) Of course, in some respects the most important tension is our built-in expectation that the soloists need to begin soloing; in this case, however, rather than a grand entrance, the violin and viola appear unobtrusively over yet another pedal tone against the soft pull of dominant harmonies.
Mozart's flair for coloristic subtlety is quite in evidence here; the soloists are merely adding two more octaves of E-flat to the existing pedal, so for their first two sustained measures they add nothing melodically, harmonically, or rhythmically. Rather, they provide a wonderful registral expanse, as if these upper harmonics of the pedal have just naturally blossomed out of the texture, which literally rises up to greet them. They become more present as the orchestra thins back downward and finally cadences into a full E-flat harmony - the way in which Mozart subtly orchestrates (in the strict and broad sense of the term) the emergence of these octave e-flats is perfectly judged, and we're left with the impression that this sonority has existed all along.
And what a sonority it is, like some better-than-possible violin which can play octaves without the usual tension one hears in a single instrument. I'd never realized, until looking at the score recently, that the viola part calls for a scordatura tuning. By tuning its strings up a half-step (B-flat, E-flat, A-flat, D-flat), E-flat Major becomes a brilliant, sonorous key for the viola in which the open strings will naturally resonate much more often than with conventional tuning. Of course, this helps to offset the usual advantage of the more brilliant violin tone.
Even though the orchestra does resolve to an E-flat tonic chord after two bars, there's an inherent tension in those floating, emerging soloists' E-flats - a tension created by the fact that we still haven't gone anywhere, the opposite of the more normal dominant pedal effect where we're aching to get back to the tonic. The prolonged tonic tension has inspired a sort of wanderlust, so it's actually rather freeing when the "violinola" (Mozart still indulging in the sweet octave doubling) descends from its pedal perch with a liquid melody, really not much more than an ornamented scale down to the next set of E-flats. From there, the fanfare idea returns to signal the beginning of . . . well, the rest of the piece with it's expected back-and-forth between violin and viola, etc. But it's that first magical entrance that I'm always left remembering and wanting to hear again.
I know it's not unusual for the soloist(s) entry to be memorable in a concerto (dozens of examples come to mind), but it's striking when one's favorite part of a work comes so soon. Other such examples I can think of are also from the Classical period: the adagio entrance of the violin soloist in Mozart's A Major concerto fits the bill for me, but even more striking is the adagio introduction to his so-called "Dissonant" String Quartet, K. 465. I think that's one of the most extraordinary pages ever written, and I'm always left a little underwhelmed by all that follows. The same could be said of the wonderfully mysterious first pages of Haydn's Creation, after which it's pretty much all downhill, a few rousing choruses aside.
As a matter of fact, my much too-wordy attempt above to summarize the beauty of 10 Mozartean seconds reminds me of reading this fine blog post by Kenneth Woods. In it, he rhapsodizes at length on the subtleties of the introduction to Haydn's Symphony No. 92. When I read the post, I was struck by two things: 1) I know that symphony well, having been taught it and taught it several times, and while I agree that those measures are beautifully written, they're not quite as awe-inspiring to me as they are to Woods. 2) That said, it's by far my favorite part of that whole 4-movement symphony. I'm sure I'm guilty of a 19th-century aesthetic bias here, but it's as if these Classical composers put some of their best drama right up front, followed by nice, elegant comfort music. (Hideous simplification, I admit.) Of course, the Romantics took that flair for mysterious scene-setting and built entire scenes out of such mystery, and I guess I'm a hopeless Romantic.