I'd stay Stalin was able to mobilize people because of the promises he made, and the fact that he (largely) delivered on them - albeit in an extremely bloody, "ends-justifies-the-means" kind of way.
For example, most Soviet citizens had understood the urgent need for industrialisation for a long time, and the diaries and testimonials we have of the 1930s suggest that despite the horrific living conditions, many people remembered the period very favourably. Plus the vast new construction projects benefited a new class of people - managers,foremen,skilled workers etc. who were suddenly in gigantic demand - who then became very supportive of the government. Sheila Fitzpatrick's 'Everyday Stalinism' covers this quite well.
I'm not sure it had much to do with Stalin's charisma as a leader, as he didn't actually turn up on radio or newsreels all that often, and he certainly didn't have any oratorical skill of note. That's why the government was able to pass an actor off as him so successfully in 40s/50s movies. The few wars that Stalin actually instigated (e.g. Finland) tended to be fairly small, and out of the population's sight as much as possible - they weren't spun as the crusades that Hitler always seemed to be launching.