The major difference between Lenin and Stalin is that Stalin was willing to expand the nature and scope of the terror used to consolidate his and the party's power, while Lenin was not. Additionally, Stalin was willing to calculate even more aggressive large-scale policies concerning collectivization.
It should not be said that Lenin was unwilling to use violence, as Lenin refashioned the application of Marxist theory to the Russian context by amplifying the violent role played by professional revolutionaries, such as himself. However, Lenin was reluctant to use terror against the politburo, but Stalin was more than willing to do so. This is exemplified in the so-called show trials of the late 1930s, where Stalin all but liquidated any possible competition, and murdered hundreds of thousands of others in what is often called the "Great Purge." Also, Lenin had not been predisposed to forcing Russian peasants to enter collectives, something he clarified in a 1921 pronouncement. Stalin, however, pursued a ruthless policy of peasant collectivization, and when many peasants resisted, he oversaw a period of intentional starvation that reaped around 7 million lives.
Finally, whereas Lenin still appreciated Russia as leading an active vanguard in the process towards worldwide communist revolution, Stalin adopted a more pragmatic, parochial approach, where Russia had to operate in isolation, at least for the time being. This almost nationalistic communism proved even more necessary when the Soviet Union found itself facing the Nazi military menace alone on the Eastern front in 1941, and when every point of mutual Russian identity and inspiration was needed to defeat the enemy.