I actually Googled the title "confidence is sexy" and discovered that various people have written about this very same topic, though not (as far as I know) with reference specifically to classical music. I have to admit that the word "sexy" is a bit beyond the scope of what I want to write about, but it was hard to resist making the statement so directly.
Which almost goes straight to my point: when we express ourselves (musically, for example) with confidence, it always sounds better than when we express ourselves with doubt. This is true even when people say something ludicrous (musically, for example) but do it with assurance; and it is sadly true when people who have all the right reasons to play a piece a certain way do it without sounding quite sure of themselves. I knew a guy in college who always gave the impression he knew the answer to every question. He was, in fact, an intelligent and knowledgeable person. But whether or not he knew the right answer, he always *sounded* like he knew the right answer, and in many contexts that was good enough. One day, however, I happened to hear him in the middle of a conversation "...now Mozart, who was born in 1741..." and I realized something. He made that erroneous statement with the same unblinking confidence with which he said everything (including the times he was probably right - mind you, he was not a musician, but a Physics major), and for everyone else (not musically knowledgeable) he sounded like an expert. (Mozart was born in 1756).
Now when it comes to a statement like that, there is a right answer (1756) and a wrong answer (1741). But when it comes to a musical performance there are many things which are frankly not right or wrong but simply more convincing or less convincing. And nothing bores me more than a "correct" or "dutiful" performance of a piece of music that lacks conviction.
I don't think this releases me or any performer from the necessity of exploring, in as much depth as possible, the intentions of a composer, the conventions of his musical era, etc. The most convincing and best performances will, in the end, be those which best bring Beethoven's or Chopin's music to life, and I know that Beethoven and Chopin were much greater musicians than me. I know I will be more successful when I try my best to play their music "their way". (This is easier said than done - figuring out "their way" is in many senses my whole life's work).
But I have seen some musicians, some very fine musicians, play in too "reverential" a way, as if the music were a museum piece to be treated only with laboratory gloves in an airtight room. You cannot play Schumann or Tchaikovsky without getting your hands dirty, so to speak. The music is not to be admired only, but to be loved, sculpted, caressed.
I am not an expert on pop music, and I don't listen to it very much. But recently I was listening to the radio and had some observations. I have noticed that I find famous women who sing pop music much more attractive (sexy?) than they would be if they weren't singers. Of course this is in part because (despite what you may think of men's interest only in women's looks) it is more sexy to be successful than to be nobody. But specifically in pop music, I think the the singers express thoughts and ideas that most people, in their everyday lives, are a little afraid (*lacking in confidence*) to say: sentiments as gentle as the Beatles' "I want to hold your hand," or as over-the-top as R. Kelly's "I don't see nothing wrong / With a little bump and grind." (Haha, C.S.K., I know you are reading this and laughing). But aside from the lyrics, it is the assured stage presence and vocal projection of Christina Aguilera or Aerosmith that make them successful with the millions who buy their CD's, the people who perhaps wish *they* had the guts to say, publicly, what these singers sing about, out loud.
Where I am unsure (uh-oh, that wasn't a very sexy thing to say!) is how to draw the line between confidence (attractive) and arrogance (not attractive). Personally, I am annoyed by the hip-hop performers who seem to focus 90% of their lyrics on saying how incredibly awesome they are. But some of these are quite successful (just not with me). I can say more definitely that I cannot think of even one successful musician who sounds reticent on stage - he or she may experience fear or self-doubt or shyness in "real life," but on stage it is like Siegfried slaying the dragon - no worries, no doubt, no problem.
But here he his on stage - with that confident swagger, no wonder the girls go nuts!