A culture is as good as the stories it tells itself. Norse legend does not feel at all like Greek myth, but they are both compelling. English culture and myth took a huge leap forward with le Morte d' Arthur and The Lord of the Rings. You can do an interesting comparison of English and German culture by reading TLotR and Nibelungen side by side.
All real world cultures are "good cultures" from this thread's point of view because they have an established myth and story that explain the mysteries of life (How to face death; How to honor sacrifice; etc.). A good fantasy culture needs the same thing.
However, most fantasy authors (whether in a novel or campaign setting) don't want to present an exact replica of a real Earth culture. So, you have to change some stuff; maybe some new stuff, maybe some mix and match. But what's important is for the myth and legend of this new culture to match up with its new appearance. That's why Robort Jordan's world is so nice - the Myth of the Aiel (by which I mean, the stories the Aiel tell themselves to explain "What it means to be Aiel") well matches the wasteland warriors you meet in the story. Same for each of the cultures in those books. Even if The Two Rivers seemed a Shire-Without-Hobbits at first, the reawakening of the memories of what it meant to be "of Manatheran" and the rise of Perrin Goldeneyes put lie to that.
Further, cultures are not one-dimensional. You can have a factor or three which dominates a culture (as Water, Spice and Shai-Halud dominate Fremen culture), but within that there are many reasonable variations. Is the culture polygamous, monotheistic and vegetarian? Without going all Petal-Throne it's important to show that you've given some thought to the less important choices the culture has made, and not just the one or two "big ideas" that shape the culture. It's window dressing, but that doesn't make it unimportant.
So:
1. Some "big ideas" that shape a culture - whether geographical, historical or religious.
2. Twice as many "small but distinct ideas" that don't conflict with the big ideas.
3. Some good stories/themes/meanings that tie the culture together. You should be able to answer questions like "What does it mean to 'be Akrasian'?" How to they live, fight, die, work, love, marry, eat and laugh?
After that you can fill in small details like tech level and magic use. Frankly, they just don't matter that much. Because from a cultural point of view, a Samurai with a katana is 99% the same as a Samurai with a light saber or a +2 Flametongue. If you don't know what I mean by that, rent and watch
Ghost Dog: The Way of the Samurai.