They copy computers, cellphones, cameras, video players, watches, bags, and everything man-made—but did you know that the Chinese also copy entire combat airplanes? It's simple: They buy one and measure every part and system to clone it.
Lately, I've taken note of both ASmartBear's It's not a Competitive Advantage and Sachin's assertion (can't find the quote right now) that when investors ask him about defensibility, he replies that they don't understand the internet.
If they can copy a fighter jet, they can copy your damn website.
What's missing from a copy? I'd say intent is one of them. Especially if your product iterates and always tries to do something hard. It'll take them longer to copy, and they'll always be on the defensive. For actual competitive advantages, check out another list by the bear.