When Nas speaks about “sleep [being] the cousin of death”, Genius tells me the line is ‘perhaps inspired by the Talmud (which tells us that sleep is 1/60th of death) or the Iliad (where Hypnos and Thanatos – i.e. “sleep” and “death” – are described as brothers)’.
is more like an example of a bad annotation. Really, we're supposed to believe that 19 year old Nas was referencing the Talmud? This is basically the stereotype of Rap Genius, that it contains a bunch of contrived ridiculous explanations for the meaning of lyrics that are very unlikely to be accurate.
Another problem on Rap Genius is that sometimes the annotations are more jokes than explanations. Judging by the downvotes people get here for posting Reddit-like jokes, I think we are all aware of the near-universal problem of jokes being upvoted over "real" content.
And one more comment...
Annotations work particularly well in two contexts; first, when explaining the meaning of a single word
I don't know if that is true. It depends on what assumptions you make about the reader. Obviously it would be bad to annotate every single word with its definition, that would be overkill. But where do you draw the line? There's no universal answer. For example, I'm sure that many people have heard the word "ratchet" in rap songs without knowing what it means, but if you look up songs with that word on Rap Genius, the annotation is most likely not a definition. Probably either no annotation or a joke, since contributors to Rap Genius all already know what "ratchet" means.