How to define a good coffee...
One helpful trick when discussing good vs. bad coffee is to think of coffee like wine. Sure, wine and coffee will both get you buzzed, but you can also taste big differences if you develop a palate.
Consider the following:
• Most people who drink wine have personal preferences about what they like and what they don't. I may not like even the most expensive and rare ice wine, for instance, because it's so sweet.
• Roughly shared standards of good and bad wine exist even in the face of personal preference. There is a whole industry of professional quality grading and rating for wine. There are also wines that almost everyone would say taste bad.
• A lot of people will drink cheap, terrible wine and be perfectly happy.
• To really learn what good wine is and what you prefer, you just need to try a lot of different wines.
All of the above is also true of coffee. In both coffee and wine, a lot of people will spend their whole lives not really knowing or caring about the difference between good and bad. At the other extreme, there are professionals who actually do side-by-side tastings called coffee cuppings.
So how do you identify good coffee?
Most coffee tastings use these terms like these to describe the different attributes of coffee:
1. Body: also called “mouthfeel”. This is the tactile feeling of the coffee in your mouth. To oversimplify, bad coffee tends to be at the extremes here, i.e. it's extremely watery, gritty, too oily or viscous. This is in large part based on the amount of solids and oils extracted from the coffee grounds, so brewing method makes a big difference here. For example, pour over coffee brewed using a device like a Chemex typically has almost no body, because the water passes through the beans only once and at low pressure.
2. Sweetness and acidity: great coffee has a balance between sweetness and being acidic. A lot of really bad coffee is overly acidic when brewed strongly. There can also be other other undesirable tastes like being extremely bitter or alkaline. Medium or lighter roasts tend to be sweeter than dark roasts, which among independent roasters have fallen out of favor for most purposes except espresso blends.
3. Flavor and aroma: this is the most subjective, wine-like part of coffee tasting, in part because it's based largely on smell. There is actually a wheel of flavors from the Specialty Coffee Association of America. If you get too deep into this you'll end up sounding like everyone's least favorite wine snob, but that doesn't necessarily mean it's bullshit.
4. Aftertaste or “finish”: basically, how long does the taste of the coffee linger in your mouth? Is it pleasant or unpleasant? Good coffee shouldn't make your mouth taste like old cigarettes and shoe leather after you drink it. Likewise, the flavor shouldn't just disappear immediately.
You can develop a coffee palate by experimenting with brewing methods and different coffee roasters (companies like Blue Bottle and others will even do home delivered subscriptions). If you're lucky enough to live in an area with local roasters and cafes, get out and try places!