The details (like weighting factors) of the credit scoring models are considered proprietary information, but there are general principles that they all use. Some of the factors they take into consideration are:

1. Credit history. For how long have you had accounts in good standing? If you have a relatively short history--i.e., you are a new borrower, this is less preferred than someone who has a long and extensive history of responsible borrowing. Metrics used include the average age of credit and the oldest open account.
2. Total amount of credit. How much have other lenders been willing to lend to you? The more, the better, assuming again, no derogatory marks or late payment history. How many open accounts do you have?
3. Types of credit. Credit cards, car loans, and home loans are all different. If you can handle more types of credit, the better.
4. Utilization. If you don't use a lot of credit relative to how much has been extended to you, this looks better. But because utilization can vary from month to month, it also does not have a long-term effect on your score.
5. Hard inquiries. If you have recently asked for a line of credit, then this counts against you. The impact ranges; typically it's small for a credit card and a bit larger for a home loan. The effect drops off after a few years.
6. Late payments and delinquencies. This is a huge one. Even a single late payment can torpedo your score. Defaulting on debt is enough to drop your score by hundreds of points.

There are other criteria but these are ones most people know about. In your case, it doesn't sound like you have much of a credit history; you have relatively few open accounts. Early payment of a loan is not necessarily to your favor because the scoring models really like seeing open accounts with regular payments on time. In the absence of any derogatory marks, these would be the main reasons for not having a score in the 800+ range.

Contrary to popular belief, 0% utilization is not a bad thing. Any potential hit to your score for being at 0% instead of, say, 1%, would be tiny. The actual issue is closing out any open accounts by paying off the loan. Another issue is lack of activity on revolving lines of credit: the card issuer has expenses related to keeping your account open, and not using the card means they don't get the merchant fees. So this might be a source for the misconception that 0% utilization is bad: people confuse utilization with inactivity.

In closing, I want to emphasize that whenever we look at credit score, it's just the result of a model that tries to weigh all the information in your credit file. A prospective lender is not going to just approve you solely on the basis of your score: they will look at your file. The score is just a convenient way for them to get a quantitative sense of your creditworthiness relative to other borrowers, and many lenders use the score as a way to make quick decisions--a good score is like a foot in the door. They can still deny you if they see something in your file they don't like. Focusing on optimizing the score is missing the point; the file is what really matters.