There are a lot of differences, maybe hundreds of differences if you took both apart and compared each component part rep to gen.
But, from three or four or five feet away they will look "identical" to 99.9% of the people who see it and that is remarkable.
Remember, replica manufactures are not trying to manufacture gens, or even anything very close to a gen "part for part." They are just trying to replicate the look of the gen as best they can for a certain price point. How can anyone expect a rep that costs less than one tenth the price of a gen to be anything like the gen part for part? It would be impossible. I am sure the Chinese could produce a 99% true part for part Rolex for half the cost of a gen Rolex or maybe for one fourth the cost of a gen Rolex if they wanted to, but most reps we buy cost us around 1% of the cost of the gen. When we learn enough to obtain the better reps we are getting a really remarkable deal.
Lets be reasonable in our expectations here.