The attentive barista grinds, doses, and delivers a level tamp. He brushes the rim of the portafilter, and flushes the head. You look on from the other side of the counter, content to watch him work. He knows what he's doing, clearly, and so you await your demitasse. You see a trickle of gold, and then a stream, dispensed in to waiting and preheated ceramic. Well done again. The shots cut off at 30 seconds, and he hands the vessel to you with a detached smile. "Thanks."
The crema is unbroken, and your expectations rise. You smell tart cherry, a dusting of cocoa, and roasted nuts. You sip... and find it acerbic, a bit flat. You try again, and again are met with the taste of aluminum foil. Curious. Everything went so well - or so it looked. Perhaps your taste is maladjusted. Perhaps that curry was a bit strong, and your tongue just hasn't recovered. So you skip the tongue, and bomb the remaining ounce down your throat.
Not satisfied, your order a small coffee, to go. "Light or dark?" he asks. "Where are they from?" you respond. "The light is Yirgacheffe; the dark is our house. It's a blend of South American beans." You take the Yirgi, and smell lemon peel floating from the cup. This will be better, a palate cleanser. You drink, and taste over-steeped Earl Grey. The lemon is tangy, the acid too aggressive. Something is... off.
You leave the shop, cup in hand, thinking just a bit more on the matter. These things do happen, after all. But why? The barista looked like he knew his way around the bar, like he knew how to pull shots. And the coffee is hot enough, probably fresh-ish, at least. You grow a bit more confused. The barista certainly had the appearance of knowing how to make coffee.
I interrupt this hypothetical now, to point out that said barista may well have an honest appearance. He may indeed be very attentive in his coffee preparation, cross his t's and dot his i's. But there these are necessary, not sufficient, conditions. Espresso and drip coffee can be brewed at the correct temperature, with the correct dose, and still yield a suboptimal product. Probably, you've wondered on this. Probably, you've had good reason to.
The cause of our problem here is simple, but too often overlooked: Rancid oils. Even the most dedicated drink crafter can get lazy, when it comes time to close. It's late, you're tired, and the sandwich shop across the street is closing soon. You dump the pots, rinse the portafilters, and jet. Those oils do not leave, however. They stay, and they accumulate. One skipped cleansing is not significant. But too often, once become twice, and multiplies from there.
Thus anyone who would claim to be a good barista must also be a good maintenance tech and custodian. This is not the glamorous part of the job - insofar as any part of it is. There are no tips for this, no "Oh that's pretty!" or "Oh my god, this is delicious." This is thankless, no one will see, and really, no one will be the wiser if you skip. But you don't, and you can't. The best barista in the world is shit if their equipment is, and rancid oils turn any pot or portafilter in to exactly that. You're never too good to do the things necessary to make good coffee.