Since we're on the topic of rap and rock, I thought I would add my 2 cents.
If you claim to be a musician, you either better play a musical instrument or use your voice as a musical instrument. Music is very vaguely defined, but when I took musical composition classes in college, if we wrote a piece we had to defend it and prove that what we wrote was music and argue why it was good.
When I look at musicians that don't play musical instruments, I have to judge their musical skills by their lyrical content or the strength of their voice. With someone like Celine Deon, Steve Perry, Freddy Mercury, Mariah Carey and the likes, it's easy to tell that their voice is an instrument given by the graces of god. Their voices are incredible. They could write total crap lyrics and I would still be impressed.
Keeping that in mind, consider the voices of DMX, Nelly, Lil Wayne, Soulja Boy, TI, and any other rapper that kids are listening to these days. They have no vocal range. They can't hit a F sharp in a really high octave. They probably don't even know the chromatic scale. So they better have some serious lyrical content to make up for their voices. I'm going to pick on Soulja Boy because he is the easiest. Consider these fine and meaningful lyrics:
"Super Man Dat Hoe
Then Watch me Crank Dat Robocop?
Super Fresh, Now Watch Me Jock
Jocking On Them Haterz Man"
One of the objectives of lyrics is to get a feeling out or express an oninion. What was Soulja Boy trying to express here? He wants to super man some girl and crank dat Robocop. Ok I'll stop there. Obviously the lyrics are total trash like the majority of mainstream hip-hop today. It's interesting to know that the majority of hip-hop "artists" don't even write their own shit. Some fat bitch on a piano writes most of this crap. Their is no instrumentation to back up the lyrics. There is massive bass shitting out low frequency notes and maybe some other random noises in order to make the beat/rhythm easily rememberable.
I wouldn't say that all rap lyrics are meaningless. There's obviously good rap and bad rap just like any type of music. Tupac is one of my favorite lyricists. The majority of his lyrics have strong meaning and he wrote his own music.
Mainstream rap/hip-hop is pretty laughable at best. It's nothing more than beats and rhythms that people can dance to in a club. That's really the only purpose I see for the majority of it. Most of the lyrics objectify women and only convey how much money the person with the mic has and how well he is spending it on super cars that he doesn't know anything about. In short, I don't like rap too much with the exception of a few artists.
