This is an excellent poll. In voted for the yeast and grain flavors. Any mediocre bourbon can sit in the barrel for 15 to 20 years and get those flavors, but a bourbon with the complex yeast and grain flavors can be interesting at every stage of the process. It will be flavorfull at 4 years of age. At 8 years it will begin to add depth to those flavors as the wood real starts to take hold. After 12 years The complex caramels and vanillas will begin to dominate, but those yeast spices and fruits will still be there to provide a supporting role and at 16 years or older, the barrel will cominate, but somewhere in the background, those grain and yeast flavors will still be there refusintg to go away.
The prime example of this is the difference between Stitzel-Weller bourbon and the weaker flavored Bernheim distilled Weller and Old Fitzgerald products. You can really tell the difference because the product had lower distillation and barrel proofs at Stitzel-Weller allowing for more grain flavor. You can add to the comparison when you throw in Van Winkle distilled Stitzel-Weller when they used jug yeast instead of dried yeast.
"Our people live almost exclusively on whiskey" - E H Taylor, Jr. 25 April 1873