Do you have a beautiful custom design you want implemented? Want code that will stand the test of time?
There is a sea of developers out there. So what separates a good developer from the rest?
Communication - The most important skill has nothing to do with actual development. You need someone who can take a requirement, understand it, then implement it correctly from end-to-end. You need someone who can figure things out on their own and only ask questions when absolutely necessary.
Quality - Paying for someone who understands best practices, code architecture, and organization will pay off tenfold. You need code that is highly modular, efficient, and extendable. In other words, code that saves heaps of money over time.
Branding - You shouldn't expect a developer to be a graphic designer, but they should have an eye for design. This includes an intuitive UI for your users, consistent fonts/colors to convey your message as intended, and logical responsive designs that don't leave important information out.