How does Minecraft compare to second life? I suppose the main difference is fixed physics rules? Meaning there is wood that can automatically burn - in SL you could create something that looks like wood, and then make it burn with an extra script.
i don't know enough about SL to make a good comparison, but here's an attempt:
you can die in minecraft, there are enemies (in the non-free version), physics rules are static, there are no scripts, and its not massively multiplayer.
I'll add that there also aren't people in weird animal costumes having virtual sex with each other in Minecraft.