Very simply, I think what separates someone from being a hipster wearing an old watch and a “watch guy”, wearing an old watch is the irony or anachronism of it.

My impression of hipsters wearing an old watch is because it’s “old” or “retro” or “harks back to a simpler time”. Much of that is bs if you ask me though. But on a whole I think they tend to be less interested in the quality, design, or abilities of a vintage watch or what it once represented and more interested in the fact that it is old or at least older than themselves.
Although maybe I'm being harsh on my fellow gen y's because I like lots of old stuff. To be honest I bristle a little at the notion of being called a hipster because I like old things. If liking old things is the definition then I’ve been a hipster since I was about 2.

Eddie