One of the things about Greek myth that, to my limited knowledge, is different from Norse myth is that Greeks took an active interest in categorizing and cataloguing their myths. Apollodorus, for instance, wrote a short-hand collection of Greek myths that are otherwise lost, generally known as the Library of Greek Mythology in English. These summaries give the titles of the lost works and the main plot points. Hyginus was a curator of the Palatine library in Rome and also wrote such a collection, the Fabulae.

There's a whole collection of poems about the Trojan War that are lost or only survive in fragments. We know the details of the plots from these kinds of summaries. They help us understand a lot of vase painting that tell stories, but without the context of the poetry, those stories would be difficult to reconstruct.

Achilles and Penthesilea comes to mind. The earliest form of that love story and tragedy is, to the best of my knowledge, only contained in these summaries, though the story was adapted by later poets such as Virgil.

Anyway, it was the Greeks' interest in their own myths that preserved what we have of them. That's not to say the Norse were less interested, but the Greek habit of writing is what made the difference.