Even though Minecraft does its share of multithreading, more so now than it used to (I count 15 threads running when you're in a singleplayer world), it's not really a cure-all for performance issues, because the problem now becomes synchronization. Sometimes one thread has to wait for another thread, either to get its result or to wait on it to stop messing with something that it also wants to mess with. Depending on the tasks, how they interact, and the person's computer, you can actually notice worse performance or more wasted CPU cycles than if the entire thing were just single-threaded. I think this is partially the case with 1.8 currently, but I have no way to confirm it at this point.
You can't really use the game's code for any other projects, though, if that's what you mean. Even after being decompiled and deobfuscated, it's still Mojang's property. And to be blunt, Minecraft is such a mess now that I would recommend against anyone trying to clone its structure these days. Given the amount of code growth compared to the amount of features added, it's just plain bloated. Just to give you an example, there's an enum class for the bed which has the sole purpose of specifying "HEAD" or "FOOT". That's it.
Another problem that we may face, or at least something that I've noticed for myself, is that when I tried to make my own voxel engine, I found myself mimicking a lot of Minecraft's structure and naming schemes, since that's just ingrained in my head as the logical way to go about doing some of this stuff. I realize it's still my code, but I wouldn't have designed it that way without having knowledge of Mojang's code, so I'm not even sure of the legality of such a thing.