I actually often go back to leave feedback, especially on stories in my fandom (due South). I think it's partly a factor of not listening to huge amounts of podfic, since I'm not into any of the large podfic fandoms like SGA, J2, or SPN. If I listen to fewer stories, it's easier to remember to go back and leave feedback. I also have more of an incentive to leave feedback within my own fandom, because then it's within my local fannish community, where I might already know the person who did the recording. And if I don't know them and they're just starting out, I want to encourage them to do more. Also, it might be easier for me to leave feedback since I'm a podficcer and I know what I'd want to hear feedback about.