"Good and bad coffee beans" is a function of several things; origin, processing, roasting, proper storage, brewing and..., most importantly...personal taste. That said, here are a few of MY thoughts.
We prefer "single origin" beans rather than blends because the characteristics seem to be more consistent over time. Then, proper processing (hulling, washing, drying, sorting, etc.) has some effect on the beans. The biggest factor, however, is in the roasting.
The roasting level is what determines the final flavors in the brewed beverage. Some beans take well to dark roasting and some don't. Over-roasting produces bitterness; not desirable!
Finally, proper storage determines how long the bean flavors remain at their peak, even when vacuum sealed. As beans age after roasting, some of the delicate flavor nuances are lost.
WE like, single-origin coffees such as Ethiopian Yirgacheffe and Sumatra Mandheling. Some of the Central/South American blends are pretty good too, but we find them somewhat inconsistent in flavor due to changes in the mix.
[EDIT: Of course, I got the flavor profiles of Yirgacheffe and Mandheling reversed! "Of all the things I have lost, I miss my mind the most." I have corrected the text below.]
Mandheling (not Yirgacheffe) has a nice, somewhat chocolatey nuance while Yirgacheffe (not Mandheling) has just a hint of citrus and a little earthiness. Both are full-bodied in the medium roast version when brewed properly.
As for additives, I NEVER use any sweetener (sugar or artificial) while LadyDi sometimes adds just a touch of it. Until I had my gall bladder removed a year or so ago, I ALWAYS had my coffee BLACK! Unfortunately, my new, abbreviated "plumbing" system no longer handles acid (even small amounts) well, so I add a splash...just a splash...of cream. In my Spanish-Basque heritage, that is known as "Cortado" or "Kafe Ebaki" ("cut"). LadyDi likes one of the flavored "creamers."
The bottom like is to add what you like (or need) to make the brew taste just right...for you.
I'm sure this is far more than you wanted to know.
TJ