There are a lot of variables and specifications that can come into play when choosing a grinder, but in general for a good grinder, you want one that will do a good job grinding with a small grind size (espresso grind size is quite small, a cheap grinder will struggle and wear out quickly), the ability to fine tune the grind size (for each bean type and age it's different), the temperature of the bean while it's being ground(you don't want the burrs to heat the beans or the temperature of the motor running to heat the beans), and burrs that deliver particles ideal for espresso (conical vs flat burr). For ease of use, there are things like how you are going to determine the amount of espresso being put in the portafilter (doser or electronic dosing?). In order to ensure freshness and a lack of waste, you will need a grinder that doesn't keep a lot of grinds in the burrs after you have stopped grinding, as the next time you grind, these excess grinds will end up in your portafilter. These grinds start going stale within 7 minutes of grinding, so the flavor of your drink tomorrow can be altered by the grinds in the chamber from today. Grind size consistency is one of the most important factors in producing quality espresso as ideal extraction is different for each grind size. In an ideal world, you want a grinder that produces particle sizes that are identical. While this is essentially impossible, there are grinders that get close, such as the Mahlkonig EK43, which is why people pay top dollar for this grinder.