Having a good story that fundamentally works. If the story isn't in good working order, the query isn't going to work. When our story didn't work (and I didn't know it), we went back and revised the query, trying to find the magic piece that would make it work. During that time, I'd see ones by published authors who got accepted, and the queries seemed so ordinary! No gimmicks, seemingly nothing special. But in all of them, it radiated through the query letter that this author had a story that really worked. Our story was convoluted and confusing, without a focus because of setup problems, and now I can see how much of that was obvious to the agent just in the two paragraphs of a query letter.
I don't think there's any one important thing. If the agent loses interest and puts down the letter at any point, then the query lost importance to the agent at that point. It could happen in the summary, the credits, or even the word count. The goal is to get him to the end of the letter and asking to see more.
As to credentials, think about why you're the person to write this story. Try writing about that. Or some element of your background that applies to the story--and your audience. You might also consider joining a professional writing organization that you can list in the letter; not all of them require professional publication to join.