I kinda feel the same way, in that I can't really tell if a lyric is any good without the music to go with it.
With that said tho, I think a really well written lyric should say something that reaches out to people. Not necessarily direct or anything, but having some kind of hidden meaning and a really good song structure.
If it's a tune you want to get on the charts or sell to a publisher, it's gotta be about a subject that's universally understood (love etc..), you have to stick to one viewpoint be it first, second or third person. Think about the setting and whether the story is one point in time or a memory or moving in time.
Ya gotta be able to sum up the whole song in one sentence if somebody asks you to describe what it's about. The title has to be memorable and if you really want to make money, the song should cross over more than one chart.
If it's got all that and a good hook, a decent singer and is cleverly written, ya should have a decent song.
Of course there's no real hard and fast rules, but it's a good idea to know what they are before you break them.