Eminem's greatest asset is PASSION.
Notice his weakest albums (which were still better than most of the albums released in that time period): Encore and Relapse. I agree Encore was a catchy album, but the lyrics were simplistic and most of the songs weren't meaningful. Encore was probably his least controversial album at the time too. Encore didn't have much passion because Em was already the most famous rapper in the world and wasn't as hungry for success. Relapse had great lyrical prowess, but even though he was using a lot of words, he really wasn't speaking much from the heart. And I doubt those horror songs were Slim Shady, they were just detached ramblings.
His most acclaimed albums: Eminem Show, Marshall Mathers LP, Recovery all have in common that he had a lot of passion in the album.
He wasn't just staying at one spot and coasting. He was actively trying to push new ground and make a point. Whether it was to show where he came up from, how the media treats him, what he apologizes for, how he feels about his family, about fame, his best songs were always when they came from the heart and were driven by deep emotion. Even some of his funny songs were driven by actually passion, and they had emotional undercurrents. Relapse, Encore (and Re-Up), while they had a lot of assets, they didn't have passion.