Intellectual property is a difficult thing. There's no concrete formula for it, only that it's obvious when it's really bad or completely irrelevant.
Code is nearly always criminal to copy (though, of course, some things are just mathematically identical). textures and art are sometimes okay if they are obvious references. Mechanics and gameplay ideas are the real "meat" of the argument, because they are a lot more universal and dependent on context (It's usually clear when someone outright copies)
You could of course, just be polite, ask for permission, give credit, and apologize if you offended someone. Maybe it works like that on the other side of Vazkii's portelf.