I think the primary factors differentiating pipes are the drill/draw and stem construction. I prefer a wide, thin mouthpiece because i primarily clinch. The drill should allow a pipe cleaner to easily pass. That is a complete dis qualifier for me as well. And there are certainly subtle to even dramatic differences in the quality of the briar used.
With all that said, I also agree with Embers in for the most part, more of the differences are in the Smoker's head than reality would dictate. The reality is however, that cosmetic and emotional issues/feelings come into play with most things in our life. The cars we choose to drive, the food we eat, the drinks we drink, the friends we have and even the woman we marry. It would be naive to think similar issues don't come into play with enjoying one pipe over an other.