Things that taste bad in coffee might be:
- dries out your mouth
- watery/ tasteless
- really bitter
- tastes like wood
- smokey/ burnt
FYI, it's hard to pick up on these taste characteristics if you drink coffee with cream and sugar, so if you want to do a taste test, try it black and at a warm (not hot) temperature.
For more information on the sensation of taste and all it's myriad complexities, you may want to check out these resources:
http://www.ucdenver.edu/academics/colleges/medicalschool/dep... Sue is a world expert on taste, along with Tom Finger, and her work is excellent in every way.
https://www.amazon.com/Principles-Neural-Science-Fifth-Kande... The bible of neural sciences, though thick and dense, the sections on gustation and olfaction may prove useful to your endeavors.
Edit: Here are some other resources on how to modify your taste sensation to better understand how your taste buds work:
There is also a supplement that you can take that will elliminate your sensation of sweet for a few hours, but I forget the name
Lastly, to understand umami, try getting a few bags of chips of similar salt content and hold your nose the entire time. One of just regular Lays potato chips, one of something like Doritos, and one of those baked Parmesan cheese ships from whole foods. While eating them with your nose held close, try to recognize the changes in the umami taste.