Which is why he's the defining artist of the generation. Decades from now, just like people associate the 50's with greasers, the 60's with hippies, and the 70's with disco, the stereotypical view of people in the early 00's will be college bros and Paris Hilton-types. I can't think of any artist who appeals better to that demographic than Eminem.
You don't necessarily have to be good to meet the requirement set in this thread's title. I think the fact that those who in the past have defined their generation (Dylan, Jackson, Cobain) were respected, critically acclaimed artists is rather incidental, and maybe says a lot about the tastes of prior generations compared to this one.