Flavor – Why does pistachio ice cream not taste like pistachio nuts

Main reason that comes to mind is, for the same reason compound butter's flavour improves over a couple of days: infusion.

Fat can be infused with flavour, and holds flavour incredibly well, but it needs a little time (viscosity of fat vs water). Otherwise the fat in Serrano Ham would taste just like it would on day one.

By holding your mix/custard overnight you are allowing the coffee or vanilla flavours get infused into the mix. You can somewhat accelerate this by letting the vanilla sit in hot ice-cream mix for 1hr as suggested by many pastry chefs (e.g. Simple French Desserts, Jill O'Conner). I suspect the acceleration is due to the lower viscosity of fat at higher temperatures. The taste does follow the physics here were the higher the viscosity the longer infusion (read equilibrium) takes. Serrano ham: two years. Hot deep fryer oil: seconds.

By the way, when it comes to ice-cream, they also recommend aging your mix/custard for up to 48 hours. It may be worth the experiment to sous-vide your ice-cream batter/mix at say 55C for a couple of hours and compare taste with the over-night version.

Although mouthfeel may play a role in this case, the 'flavour improves over time' effect is present even when crystallization is not happening.

Why do you think it was one single thing you did? It turned out good because the whole process was within the parameters which produce a good ice cream. A single mistake can ruing an ice cream, but a single right thing cannot make an ice cream.

so I can repeat this with recipes that aren't that one, not intentionally making 'mistakes'

If a good recipe is followed correctly, it will turn out this way without the need to do anything else. An ice cream which gives bad results means that either the recipe was not good enough, or that you made a (real) mistake somewhere while making it.

There are no magic tricks in making ice cream, some little thing which turns any strange mass into a fluffy wonder. There is expertise, and knowing why each step is done a certain way. Once you know this, you will be able to fix bad recipes by correcting whatever is missing. But this knowledge is too much to fit this paragraph. Basically, you need to internalize some simple ideas of food chemistry (such as fat globules getting in the way of proteins trying to coagulate), then learn the basics of egg chemistry and custards, and then learn about ice cream and freezing. Reading the chapters on eggs and dairy from McGee on Food and cooking should be sufficient. But we cannot write up these couple of hundred pages here in an answer.

You can also read our other questions on crystalizing ice cream, they have short guidelines. I found this one with a quick search, but there are more. The Perfect Scoop also starts with a chapter explaining some of the methodology behind ice cream, before starting with recipes. These are good ways to start, and may be sufficient for you. They won't be enough for learning how to create your own recipes, or even for recognizing problematic recipes before making them. But once a recipe has failed, they give you a few options of what to try with the hope that it can be made to work next time.