I'm referring to the inability of many rappers to convey a consistent message in a song. I've listened to rap my entire life and the subject changes so drastically, even within a song, that there doesn't seem to be any real consistency, other than the fact that it rhymes. The guy in Linkin Park, however, is able to maintain a consistent, yet angsty message throughout most, if not all of their songs. Linkin Park is good music to sing along to, because of their simple lyrics regarding break-up frustration, and the fact that the music itself, while often overly simplistic, is quite catchy.
And 50-Cent, god bless his heart, is often very inconsistent, which was my point. However, I also get your point, and don't think that consistency is the only factor that makes a good song, it's just something that I find very irritating when listening to rap music, which 90% about the lyrics.