I think of bitter to be on the opposite end of bright (tart, sour, acidic) on the coffee taste spectrum. I think bitter usually results from over roasted beans or over extracted coffee. On the extreme end of bitter, I would say it tastes burnt almost. I'm no coffee connoisseur, but I do not like bitter coffee and prefer lighter roasts.
You wouldn't believe how many factors can go into making the perfect cup. DH works at a coffee shop. He really enjoys coffee, the process of making it, the science involved, and geeks out with the equipment. (Yes, that sentence was about justifying the value to him, because this is MMM, he kind of spends a lot on coffee, but is otherwise extremely frugal. Coffee as an interest is pretty cheap as far as hobbies go, anyway...) I'm talking a $150 bluetooth smart scale that measures in 10ths of grams with response time of 20 ms that communicates with his iphone. I can't remember what the phone tells him, the scale, phone, and other equipment all work together to make his coffee exactly as he likes it. He does a pour over w/ a kalita wave brewer for himself, and uses a chemex for a crowd. (Like, a crowd of a few...)
We both drink coffee black if it is a well made cup, but find we need to add sugar/cream for not-so-well-made coffee. (Starbucks and pretty much any place that isn't a third wave cafe = sugar and cream necessary.)