So, a hint. This theme is somewhat meta. It's not a common word or idea in the titles and/or artists, and it's completely unrelated to the subject matter of the lyrics. It does have something to do with the lyrics, though.
And that's all I'm going to say. For now.
Update: the theme is now guessed! Props to the lovely and talented regann, who correctly surmised that the theme is songs in which the title never appears in the lyrics. Congrats, regann! You win, well, nothing. Except bragging rights. Bragging rights are always good, though. ;)