Okay, it's 1924, and Lenin's testament sinks any hopes for Stalin to succeed. Trotsky succeeds Lenin.
First, I want to establish that Trotsky was very much a hardliner, even though he came aboard the Bolshevik bandwagon late. In fact, it was the fear that he would abandon the more moderate policies instituted after the civil war (which Stalin would abandon later, anyway) that contributed to his actual downfall.
He would not have forced so massive a industrialization effort as Stalin, though, so Russia probably wouldn't have had the production it had to throw against the Nazis during the War.
However, he was a brilliant military organizer who's ability allowed the Reds to defeat the Interventionists. He would have also contributed more to Communist movements abroad, having not abandoned international Socialism.
I might conjecture that Trotsky could still have led the Soviets to victory. Two additional points: He would not have manouvered the USSR into a pact with Germany, thus Hitler would not have been confident of security to the east. It would not have discredited the Communist community worldwide. Also, having not so thoroughly brutalized his country, Communism would not have had the same stigma attached to it as that after Stalin's death when Khrushchev revealed the truth.
Personally, I imagine the post war would have been less confrontational. But ultimately, the same economic problems would have hurt it. But it probably wouldn't have had to feel that it needed to overspend on it's military, and likely would survive today, not really an "evil empire", but not a superpower either. Maybe a step ahead of China, but less stable.
Just as responsible for the Soviet collapse as the economic stagnation was the nationalities issue which would not have been any better under a Trosky-influenced USSR.