For me, "good enough" is when a system covers at least most of the full frequency range, with realistic soundstaging, imaging, dynamics and transparency and no obvious sonic flaws. When I close my eyes it captures 90% or more of the live-music experience.
At a certain point the quality of the recordings that we enjoy is more of a limitation than our gear or maybe even our listening rooms. I think a lot of pro recording gear, especially in times past, isn't of as high quality as audiophile gear.
I consider my current system good enough. It's mainly reading forums like this one that even makes me consider upgrading.