Overcast beats Apple's own app simply by virtue of not forgetting where you were when the app isn't opened often enough. It might have other features that are amazing, but I like to listen to looooooong podcasts, and there's just not enough granularity in seeking if you're talking about 3-4 hour episodes to tolerate losing your place if you don't listen to the same one all the time. It can't involve a massive amount of data per episode (unique podcast ID + time offset?), but Apple's app will lose your place often. Overcast does not.
Its other features are all just little extras compared to that, although they're very nice.