The quality of grind tends to be the weakest link in most home barista situations. It's easy for most people to justify buying the expensive espresso machine, or buy a kilo of beans from the last cafe they got a great coffee from, but the grinder is often neglected.

For the nerds on HN, the difference between a good grinder and junk is the consistency of particle size. There's an optimum particle size that will result in the greatest amount of desirable flavours being extracted; most cheaper grinders will spit out a wide gamut of particle size.

One of the most important jobs of a barista is to manage the grind fineness to optimise for the beans.

The reason that perfect espresso isn't made by a robot—the reason why the process is so variable—is because the source material (the beans) are volatile. They're constantly changing, each batch is slightly different, their properties change as they off-gas every day from when they're roasted, and they're greatly affected by storage and ambient humidity. Some of these variances can be compensated for by adjusting the grind.