1) A Lenin who lives will still be leading a brutal dictatorship. Due to Lenin being more secure in his power than Stalin, it is likely that the 30s will be less bloody, but exactly how much less bloody is hard to say.
2) The NEP will end and some sort of collectivization and focus on heavy industry development will happen. In fact, with Lenin still alive and healthy, the Bolsheviks won't be distracted for the last half of the 20s with power struggles, that could actually see the NEP end sooner. That said, small changes to the timing and how these things are executed can lead to radically better outcomes. Stalin was not only ruthless about collectivization, he was unlucky. The high point of collectivization came at the same time as a drought and the worst point of the Great Depression for the USSR, meaning trade income was depressed and food supplies also depressed. Collectivizing a few years early or a few years late could have saved millions of lives.
3) As people have pointed out, Lenin being in charge can lead to very different KDP policies in Germany which might be enough to avert the rise of Hitler. Of course, in Hitler's place, Germany could instead get a military government that goes to war with the USSR with the economic backing of Britain and France.
4) I think people over-egg the "wanted to spread communism" angle whenever any other potential leader of the USSR post-1924 is discussed. The truth is, by the time the Russian civil war had ended in 1924, all of the Bolsheviks could see that the time to aggressively spread Communism had passed. The "little wars" elsewhere in Europe had ended, the worst of the post-WW1 economic crisis had passed and Germany was no longer on the brink of civil war. The truth is ALL of the Bolsheviks, including mr Socialism-in-one-country himself believed that spreading the revolution was good for Russia and good for the world. All of them were also rather worried about uniting the capitalist powers of the world against their fragile revolution. I think that without Stalin, the export of revolution would have fallen sharply regardless of who was leader, simply because all of them saw the world in largely similar ways.
fasquardon