Consider this a stream of consciousness reply. ie, I'm trying to figure out the answer as I go...
Lyrics, are a method of conveying some sort of message, typically emotional, although often philosophical, political, or some other sort of artistic topic (Note how often art is tied into philosophy, politics, spirituality, etc). So in a sense, the mark of a talented lyricist is the ability to convey such a message in a way that is both accurate, yet vague enough so that the listener can relate the lyrics to their own life experience. For example, some would say that the emotional message in some "emo" music is "I'm hurt," but the message comes across as "Girls don't like me, I'm a whiny bitch." This would be considered poor lyricism. (Disclaimer: I am not talking shit about emo. I like several emo bands. Its just the first example that came to mind. Besides, lets be honest, there's no bad music worse than bad emo).
To tell the truth I think that is a better definition of ability, rather than talent. Ability being the product of talent and dedication to the craft. The craft of lyricism involves studying a few other topics, poetic devices (metaphor, simile, etc) and the art of word sounds (alliteration, assonance, slant rhyme, etc).
In terms of Singing/songwriting, lyricists of ability are best noted by their ability to craft lyrics that support the emotional message of their music, that fit with the melody and mood of their music. The lyricists of the most ability are those that write lyrics that do not draw attention to themselves within the context of their song, but stand strong alone nonetheless.