There are many qualities that make up a successful voiceover career. There are many ways to look at the success of any voiceover artist. Different vocal qualities create different kinds of appeals.
There is a recurring quality that I've noticed over the years that has a major influence on a persons success. That quality is authenticity.
Many people are very well spoken and have learned the various styles of voiceovers. There is an issue however that in some cases the voice is stuffed into the style instead of figuring out how that voice and personality can best represent that style without going outside of the compass of the voice or personality. In other words there is a loss of authenticity. Some times casting directors will write notes like, "Nice voice...doesn't mean it". If something isn't idea driven, if it does't sound like you're speaking spontaneously to a close friend, it lacks authenticity.
It doesn't sound like a person "means it"
Exploring the dynamics of your voice when you are most authentic and natural can pay big dividends in taking your voiceover career to the next level.