I mention the X factor rather often in my teaching. It is a term I first heard a colleague at the Brevard Music Center use in reference to his instrument, a classic one with a classic tone. It gave him the X factor.
I use the term primarily to indicate the difference between a workmanlike performance–one that is good and clean but somewhat bland–and an artistic performance. What you must strive for is a performance that has a great tone with that little bit of extra style and nuance.
Another way I sometimes word it to students is that, right or wrong, there is a difference between the playing of a principal hornist and a section hornist, at least in our imagination. A good section player can and should be able to lead and match with great style, of course, but may not reach quite that level due to choices of equipment, articulations, tone color, etc.
Clearly it is something you build up over time. If you aspire to play professionally you won’t get that X factor going without playing in professional groups. Getting past sounding like a good student to sounding like a pro is a process and when you have it you have achieved the X factor.
For more on auditions and orchestral playing, see my publication, Orchestra 101. The text in this article was adapted into a section of this book; for more information on purchase visit www.hornnotes.com