As David Smith, reporter of the Manitowoc Herald Times Reporter points out in "Spill the Beans: Three keys to what makes a good coffee":
Variables such as the café décor, the music it plays, your relationship with the folks behind the counter, even the kind of cup your drink comes in, all affect your enjoyment.
As you probably know, and as Smith tells us, it isn't just the flavor and aroma of the coffee that makes the drink. The atmosphere of the coffee house, the personalities of the baristas, even whether or not you like the furniture of the shop, all play a part in your reaction to the drink.
However, when you leave the coffee house with your 100% recycled coffee cup with a functional and fashionable sleeve to protect your fingers from the delicious heat radiating through the cup, all you have left is the coffee itself to judge.
Smith's article judges the coffee on three things before it even crosses the counter to you. He says:
One, the coffee itself: the beans or, more accurately, coffee seeds.... Two, if you're getting a cappuccino or latte watch how the espresso comes out of the machine.... Three, steaming of the milk. Milk makes up the largest percentage ingredient in a latte or cappuccino, making its preparation at least as important as the espresso.
The beans used to brew your espresso should be fresh. The beans should be oily-looking and they should be ground just before they are used. Once beans are ground they become stale quickly.
When a shot of espresso is pulled, the stream coming into cup should be a thin stream. The description given is that it should look like a mouse's tail. The espresso should be brownish-red in color. Once the shot is finished (which should only take about 30 seconds if it's a regular shot), there should be a nice, thick head of foam (called crema) on the top.
When I worked as a barista, steaming the milk was a more difficult aspect to master. There is a balance between the steam and the depth of the wand in the milk and how long the milk is steamed that creates a perfect milk for use in coffee drinks. When you're standing in the coffee house, listen to the sound of the milk as it's being steamed. It should make a low, rumbling sound. When the milk is steamed, there shouldn't be a foam on top (unless you ordered a cappuccino, of course). If there is, the barista held the wand too high in the frothing pitcher, not properly steaming the full pitcher of milk. Properly steamed milk should have a creamy texture to it.
These are not, of course, the only factors in good coffee, but they are important, and if you're careful about your beans, brewing, and milk, you'll be well on your way to being a good coffee snob.