Like many successful rappers who have had the opportunity to release several albums, Eminem has a number of calling cards. (Nevermind that Em intentionally complicates finding an "essence" or "center" to his rap style with his three personas: Eminem, Slim Shady, and Marshall Mathers.)
When The Slim Shady LP was released in 1999, the rap scene was stagnant. The hip hop world had just lost 2pac and the Notorious B.I.G. to violence, and rap acts like Jay-Z, Nas, Method Man, Busta Rhymes and Outkast were churning out good music, but it was all familiar. With the arrival of Eminem, there was a fresh sound. He was first notable because he was "that white rapper" who actually turned out to be good. Backing by rap god Dr. Dre gave Eminem credibility that no white rapper ever had before.
Soon Eminem's style began to speak for itself. His raps were playful, but with dark and violent themes. He had a high nasal voice, but he was delivering incredibly complex and clearly articulated rhymes. He also had a penchant for self-deprecating themes that would have been repellent to most rap egos.
Eminem was never positioned as a gangsta rapper; given the color of his skin, that probably would have been an impossible sell. But Eminem did have a rough upbringing, and his unassuming attitude and his brutal honesty about drug use, his own vulnerabilities, being bullied, and relationship problems was in stark contrast to the champagne and diamond lifestyle being sold by other rappers.
As he released more albums, Eminem built several continuous themes into his work, including mentions of his ex-girlfriend and his mother, references to his personal flaws and mistakes, disses to other rappers, and a rejection of the celebrity lifestyle. There is a somewhat widespread perception that Eminem's style is clownish, but this would only be truly speak towards his music videos, which often skewer celebrities and showcase his funniest raps. There is a major difference between the sometimes-goofy Radio Eminem and the more serious Album Eminem, who deals with sensitive subjects like drug abuse, abandonment, and emotional isolation and then pushes buttons with fantasies filled with murderous rage and revenge.
Multiple personalities, varying styles of rhyme delivery, and a complex web of human emotion woven into his music show us that the only calling card Eminem truly has is that another card is hidden up his sleeve.