Short answer: everything. The presence, the energy, the intensity in effort, the mental, the explosiveness of the shots, the touch and on clay specifically, the way he moves and slides is like a superior art form.
Nole, a clay court style player? Hilarious, given almost all his titles are on hard (only on hard this year) and the only slam he's never won is RG. Djoko is a hard court specialist who happens to do well on clay. If 4 finals at USO and 3 finals at AO (vs 1 at RG and 1 at W), don't tell you that, then nothing will. His first master title was on hard (Miami), his first slam was on hard too (AO).