"First of all, I used to respect Will Smith. [Now] he's dissed the whole genre of rap. He dissed gangsta rap music. And that is one of the most influential musics out there. I respect him for saying his opinion, but not everybody is as happy as Will Smith. Not everybody sees life as happy and as positive as he sees it. So if he wants to rap about birds and bees and flowers, then let him rap about birds and bees and flowers, but don't dis nobody else, dude." - Eminem
So whether it was a joke or not, there was obviously a little more to it...