the whole "it's not what you're rapping about, but how you're rapping" argument.
I mean, don't get me wrong, how you're rapping/executing is the most important thing to judge the quality of the song, and great rappers can make good music about something cliche, or, well, nothing (eg. Drug Ballad).
However, I'm worried about people disregarding great concepts completely, especially if the rapping's/execution's shitty, (again, I realise that execution of the said concept is what makes it great and I'm not saying a poorly-executed song, with a great concept, is better than a song about nothing but has good rapping) I'm just saying, you should at least appreciate the great concept.