The biggest difference between a good Old Fashioned and one that is merely okay is one of mouthfeel rather than flavor. A good Old Fashioned feels full, rich, and substantial on the palette, while an "okay" Old Fashioned feels thin: it hits your mouth at the back of the tongue, all high-pitched and one-dimensional. A good Old Fashioned is a string quartet, while the just "okay" cocktail is a violin solo.
I talked to a lot of bartenders in my search for the perfect Old Fashioned. I may be overgeneralizing here, but it seems to me that not nearly enough people talk to their bartenders.
(This applies to baristas and doctors and mental health professionals and many other professions as well.)
Granted, not all bartenders -- or baristas -- are the same, but most people who put a lot of thought and effort into what they do, the people who are passionate about their craft or trade, like to talk about it. They like to tell you why they chose Cabin Still over Maker's Mark for their Manhattan. They enjoy the avenue to describe the hidden, painstaking work they put into their creations. This is not a particularly insightful observation.
But in my search for the perfect Old Fashioned, it wasn't the countless recipe variations that provided the insights I needed. It was the bartender at Parlour describing the blend of bitters they have specially made. It was the bartender at Hola Arepa walking me through how they retain the character of an Old Fashioned while making theirs with rum rather than bourbon or rye. It was the team at Marvel Bar expressing flamed orange peel over their rendition.
The fingerprints of all of these men and women can be found in my personal recipe. While my version doesn't copy any of theirs, they helped me get over the hurdles I was experiencing throughout the process (namely, adding the bass, cello, and viola to my string quartet).

- 2 oz. Buffalo Trace Bourbon
- 1 bar spoon of demerara syrup
- 2-3 dashes each of Angostura, Orange, and Peychaud's bitters
- 1 bar spoon of Grand Marnier
- 1 large ice cube, whiskey stones, or whiskey bullets
- Orange zest