Personally, I don't think that copying UI means you're copying WoW. If you copy the content, battle system, job, then yes, that's copy pasting. But woW's UI is pretty much a standard. It's just using a standard.

If you want to make a new keyboard, you know where Backspace is located, where Enter is located, where ESC is located, and you definitely expect to see CTRL ALT, TAB, CAPS LOCK. In my opinion, using WoW's UI as template is fine. It is, after all, the template almost all MMOs out there use.