I believe people register the words of songs even if they don't consciously hear or remember them. I've heard songs completely different to what I'd normally listen to and loved them without knowing why, only to find out later that the lyrics meant something to me. 'Bastards' by Defeater is a good example for me - not only did I not hear the words, I couldn't, as it's a screamo song. But on some level my mind must have recognised them.
For that reason I'll say that lyrical content is more important than people on here give it credit for. Yes, people only tend to remember the hook/riff, but that only makes the song memorable, or 'catchy,' not popular. Think of 'hi ho silver lining' up there: how many times have you seen someone actively seek it out? People only go back, time and time again, to songs that they relate to, or which put them in a certain mood, and the main ingredient in that is the lyrics.