For me, it's usually a nice design that draws me in, nice finish, etc.
Then, unfortunately, I have to see if I can afford it.
If I can, the next thing I'm looking for is a nice open airway with nothing out of the ordinary obstructing the flow.
Unfortunately, I can't assess that over the internet, so instead, I'll look at the maker.
Does he have a reputation for making nice smoking pipes.
If so, I can probably expect that the pipe I'll be buying will be a nice smoker, as well.