Experience is definitely the most important. When you are speaking with developers, look for the ones that challenge your idea rather than ones that simply do whatever you ask of them. The best developers are going to be knowledgeable about the existing marketplace and be able to tell you if your idea already exists or how it differentiates from other solutions already in existence. You want your developer to be a partner that is truly looking out for your best interest and working to help your business rather than looking for a quick profit.