A lot of ground has been covered so far, but as a pipe maker and someone who restores and resells pipes for a living, I'll put in my two cents. These are lessons I've learned over the years. Maybe they'll help contribute to the conversation.
1. Briar-You must have good briar. Age does not equal good briar. In fact some of the worst briar I've had was 70+ years old. More important is how well the briar was cured. There is a lot of crap in freshly harvested briar that must be extracted, saps, tannins and what not. Who knows if the old briar has been cured properly. Storing the briar is sub par conditions also hurts. Old briar becomes brittle and hard with age. This makes it difficult to work with. I get my briar from a reputable source and always test taste the briar from each batch.
2. Engineering-turbulence in the smoke stream creates condensation and makes a pipe smoke wet. A straight consistent diameter from bowl to bit with no kinks or burs will smoke good, even if the diameter is 1/8". The problem is comfort at the button and how to make bent pipes smoke well. A simple 1/8" hole would be thick at the stem, so the height of the draft hole needs to compress as it reaches the button. But in order to maintain consistent airflow the width of the orifice must expand. This becomes the slot and it must be as smooth as possible to reduce moisture condensation. So why do most modern artisan pipe makers use 5/32"? The reason is quite simple. When your draw is wide is creates a larger margin of error for bowls that are packed less than optimally. This is great for new smokers just learning, but a lot of old codgers who have smoked tight drawing pipes can't expect to pack the wider drawing pipes in the same fashion for a similar smoke. I have a lot of pipes in my personal collection that require different packing styles because of draw differences. The mortise and tenon should have minimal head space. All sharp corners the smoke travels over or through need to be smoothed out. The kind of chamber also affects the smoke, but as of yet, I don't know if I have a preference or if one shape or size is better or worse than another, just different. I'm pretty sure that thick walls and heat distribution make only a marginal difference. These idiosyncrasies take time to understand for each pipe.
3. Complex variables -The biggest problem when trying to craft a well made pipe is the immense number of variables involved. The briar, the engineering, the shape, bowl size and shape, packing technique, different blends are all commonly talked about. But the largest variable is what the smoker brings to the experience. How fast do they smoke? What is their body chemistry from day to day? Did they eat or drink something before or during? What are the ambient conditions such as temperature, RH, wind? Going down that rabbit hole drives me crazy, but it can make all the difference and most of it is not under the control of the manufacturer.
I'm sure I'm forgetting something, but I hope this helps drive the conversation. There are a plethora of other factors involved in what makes a pipe more enjoyable to smoke, but they may be outside the scope of this discussion.
