N 51D (= DEM L 192) appears at first glance as a nearly circular, 120 pc diameter bubble of ionized gas around the LMC OB association LH 54. A deeper look reveals a complex web of filaments and deviations from radial expansion. Using a deep XMM-Newton pointing centered on N 51D we find that diffuse soft X-ray emitting gas fills the whole superbubble as delineated by the Hα filaments. Contrary to recent findings for galactic winds, the correlation between Hα and X-ray surface brightness is not good. The X-ray spectrum of this diffuse gas cannot be fitted with the LMC abundance pattern, but implies an overabundance of at least oxygen and neon, consistent with recent enrichment from supernovae Type II. Some indications for enhanced mixing at the brightest region of the Hα shell and for a beginning outflow of the hot gas were also detected.