Having sat with my novel for almost a year, spending hours on end working and reworking the narrative, I found myself completely immersed in a world I had created. This can be a good thing. When the world you create begins to organically develop it can lead to unexpected discoveries. And they can be far more exciting than what you initially envisioned your story's path to be.
But once the story begins to take on a life of its own, it can develop incompatibilities with the original structure the author set in place. A character, for example, can start off being described as meek, but end up mouthing off far too often to be perceived as so by the readers. This will either be taken as negligence in continuity, or the readers will be confused, thinking the author is hinting at something which will soon be explained. And if it isn't, it will piss them off. Royally.
This is why it is immensely important to bring in the perspective of others, preferably the opinions of those unfamiliar with your story. Those critics/editors will be able to swiftly detect incongruencies which the author cannot. The reason is that since the conception of these characters, they have likely changed considerably, but in the author's mind it has become difficult to filter out the attributes which are no longer relevant. An outsider will be able to quickly locate phrases the character would never say and the actions he/she would never take. This serves to not only add credibility to what the author divulges about the characters, but also makes them more real.
The more people review your work, the better. While it would be incredibly fruitful to recruit professional editors in the industry, it may not be s plausible option for many first-time novelists. But do not underestimate the power of subjective opinion. Pick a representative sample and ask them to review your work. Even if your novel is geared towards a female audience, asking the opinions of male readers can be invaluable. After all, beneath all of our superficial differences we are all human. We have hearts that beat, ache, and break.
Once you have compiled critique from various sources you will begin to notice patterns in their commentary. Too many descriptions of the surroundings? Not enough dialogue? Grammatical errors? First chapter is too boring/slow/confusing?
And finally, once you have taken said critique into consideration you will be able to assess the quality of your work more accurately, and hopefully make it even more compelling.