It's cliché. But we all fear the client who actually says, "I'll know it when I see it," to set the expectations of a campaign. And as creatives, we consider anyone uttering such tripe to be absolute cretins.
But when it comes to music, we often set the same bar, even if we don't use the same words. We listen to track after track, and say we're not able to put a finger on it, but for some reason, this tune or that tune just isn't right.
Let's not be hypocrites.
Learn how to talk about music. Learn how to express the emotion you want your spot to stir. Know the difference between something that sounds organic and something that sounds overproduced. Know when it's better to use an acoustic guitar, and when to use a full symphony, and be able to articulate why. Don't expect the music house or the director or your creative director to help you through this. You need to be fluent in music. Because a great track can be the difference between a B spot and an A spot.
For starters, read some of Jim's "It's the Music, Stupid" posts here.