Yeah his best stuff is, content-wise, purely cartoonish, which isn't really a problem on a verse-by-verse basis. It's just the old bragadaccio/show'n'prove thing flipped more grotesquely and outlandishly than usual.
It's true, though, that the near-constant facetiousness of his classic music (coupled with the pop-culture references and sometimes so-so production) means that it doesn't seem as durable/lasting. I think he played up the sore-thumb aspect of him being white (the bleached hair, his voice - which has altered quite significantly over the years, the cultural references, talking about taking mushrooms and going to raves etc.) in that early stage to such an extent that his music never seemed 'serious', but really I much preferred that to the later stuff which was all self-pity, melodrama, chest-beating etc.
Going through old freestyles of his on youtube it seems like that was the perfect format for Slim Shady era Eminem (this was also true for Canibus, I suppose) - short, technically dazzling and funny verses in which nothing is really required of the rapper except showing they've got skills. Also nice to hear the hosts cracking up in the background, a bit like a laughter track in a sitcom.
''but I would like an award, for being the best rapper to get one mic in the source... everybody's pissin' me off, even the no limit tank looks like a middle finger turned sideways flippin' me off''
Anyway, suppose this is fairly irrelevant to Hip-Hop '11, except for the fact that Em still sells gigabillions of records.