Discoverability is certainly a concern, but only because there is _so much_ good content out there. I find out about good podcasts via word of mouth much more than any directory or online source.
Quality is in the eye of the beholder. I've scoured top podcast lists and recommendations far and wide, and have found maybe 5 or 6 podcasts worth listening to for me in my whole life. I'm sure there must be others that I'd like, but they're buried in a mountain of uninteresting content.