To me good coffee is consistently high quality and delicious. Lots of people have a favorite blend at a favorite roaster, or they just love Yirgacheffe. The coffees have lots of the good chocolate, nut and fruit flavors, and also lack any off-flavors, they're not too bitter or sour or have any defects. They're also always fresh roasted.
Good coffees are like apple pie, they're full of delicious flavors, and they're balanced, and they're best fresh (maybe with a scoop of ice cream too).
Every great coffee I've tried has been a surprise. Maybe they've been super sweet, or tasted exactly like blueberries or brandy or chocolate or maybe they're just so complex that I couldn't even describe it all. But they weren't perfectly balanced and predictable, and they've all been gone too soon.
Coffee is an agricultural product, and to make a great coffee, everything has to line up - the weather and soil and picking and processing and sorting and roasting. Even farms that consistently put out very good coffee can't always make a single great lot of coffee every year. And when they do, everyone buys as much of it as they can, and it's all gone in a couple months. Then we have to wait for next year, and hope there's another harvest that's just as good. Or more likely, some other farm will surprise us with some completely different coffee that tastes like nothing we've had before.
It's like going apple picking in New England in the fall, and out of nowhere you find a tree of Honeycrisps in an orchard full of mcintosh and red delicious, and this tree had the perfect weather for that variety, the perfect amount of sun and rain and cold nights. And the apples just today are perfectly ripe, tomorrow they'd all start falling off the tree, but you got here just in time...
Taking a bite of one of those apples, that's like finding a great coffee because you can't save it, it's not going to be here next month or even next week, and maybe you'll be back here next year, and it'll be perfect again? But maybe not, so you enjoy it while you can...