Trotsky would have applied greater ideological integrity, and he would have laboured for world revolution until the end. With him in charge, the mass killings would have been less prevalent, but I doubt the SU would have democratised just yet - it would have taken many more decades for that to be a viable option. In fact, we might even see Trotsky being toppled in a (silent) coup led by either Stalin or one of his clique.
As for collectivisation, that would have happened either way, even if with less casualties. That was the only way to solve the agriculture question at that time, I think.
And as for foreign policies: maybe Trotsky would have helped the German communists more efficiently, butterflying Hitler. Or maybe not much would have happened differently. It's anyone's guess.