I have experienced only two forms of great podcasts. The first is the consistent great content that spends more time with you than your close friends. The podcast is there for you, week after week, providing you with great content.
The second form of great podcasts is fleeting, but powerful. These tend to be either flops in the water, or huge cult classics. Because there is no consistent updates, the original content has to be so great to keep people talking about it even after all of the episodes have aired. S-Town one of the best examples of this I have ever seen.
With the powerhouse names of "This American Life" and "Serial" behind it, I knew S-Town was going to be good. But it wasn't until I finished it that I realized the true depth of its greatness.