In the fact that his industrialization program produced one of the quickest rates of industrialization ever. The Soviet Union under him went from a industrially backwards pariah to a superpower that would soon launch people into space. If it wasn't for the fact that he was responsible for the death of millions, he would be remembered as a rather effective, if authoritarian ruler.
It did this, however, at an appalling price and where the USSR actually did have its most advanced areas, Stalin was directly responsible for reversing this progress (the Deep Operations concept). Soviet industry made gains in a process rife with blatant lies and filled with tremendous brutality, to a point where Stalin was in practice someone with the unrestrained cruelty of an Ivan the Terrible yoked to the cruelty and malignant embrace of a new system, damn the consequences, of a Peter the Great.
The most idiotic bit of criticisms of Stalin is how little of "his" system he invented, as opposed to perfected. Stalin in a real sense is the apogee of the system built by Rurikids, Romanovs, and Leninists. And he, like them, was a bloodthirsty and cruel monster ruling a system that favored monsters. The real defect in views of Stalin is trying to view his era in a vacuum removed from everything else, he was a symptom, not the disease.