I have never been much for lyrics. There are plenty of bands that I love. I may know only about five lyrics from their entire discographies but continue to recommend them to friends. When I'm listening to a song, my primary focus has always been the instrumentation and melody (the actual "music"), as opposed to what is being said.
I've thought about the reason behind this tendency and have determined it to be the fact that it is simply a case of envy on my part -- being a musician, I must be jealous of the undue attention that singers receive when they have little to contribute to the artistic process (see: American Idol). That, and the fact that I myself cannot sing -- anyone who has had the misfortune to have heard me sing along to music in my car or, God forbid, in the shower (where I don't even have the benefit of musical accompaniment) can vouch for my absolute inability to control my voice in a way that would be pleasant for humans (dogs, I imagine, are much more receptive to my vocal stylings, though experiments with my cats indicate that they, like humans, cannot stand my voice).
Yet I understand that for most people, lyrics are an important reason why people like a song -- they identify with the various emotions and situations presented.
My experience with lyrics is that besides the chorus, my ears will pick up on one line that is either brilliant or infuriatingly stupid. For example, I encountered this when listening to the Bravery's new single, "Time Won't Let Me Go." It's a decent, if bland, pop-rock ballad that would have fit well in Third Eye Blind's mid-'90s catalog (but with an '80s twist!). However, there is one line that makes me cringe every time I hear it: "I've never had a 'Summer of '69'." If this was meant ironically, I'd be okay with this, but I'm virtually certain this was said with a straight face. We do not need to reference other terrible songs in order to convey emotion.
Contrast this with Queens of the Stone Age's "You Can't Quit Me, Baby". I only know the ending line: "You're solid gold -- I'll see you in hell." In this one line, we have the essence of the song -- the idolization of the object of the affection, in contrast to the terror that is about to occur because of the rejection by the woman. After this line, the song descends into chaos, leaving the scene to be constructed only in the listener's imagination. Simple, but powerful.
The question is then, What makes for good lyrics? There is no short answer for this, but I subscribe to the axiom of "I'll know it when I see it." The lyricist can attempt many different paths and could be successful using a variety of methods; long, descriptive and complete sentences may work in some cases, whereas for others, it may be best to use random descriptive images to convey your brilliant insight. In other words, it's a lot like poetry, which kind of sucks when you think about it. Poetry does not mix well with rock 'n roll -- it gives way too much credibility to an art form that is often vacuous, and I've yet to hear a good rock version of "Ode on a Grecian Urn."
Must good be equated with serious? I think to some extent this is the case -- I'm much more appreciative of artists that express themselves in a way that attempts to describe universal truths rather than boast of their incredible wealth, but there are counters to even that. I love it when the Beastie Boys boast about how they have "mad hits like Rod Carew," or how Bob Dylan, known to all as rock's great poet, responds to a serious question from JFK in "I Shall Be Free" with a dick joke. Fun is good, as long as it's not stupid.