Coffee

1. Coffee bean:  The coffee variety can affect the flavor of your cup. Cheap robusta coffee can give burnt tire or stunky flavors and won't contribute much towards sweetness but will give you more caffeine and crema in espresso. African arabica coffee are known for their sweetness, sometimes berry flavors an Ethiopian coffees and sometimes grapefruit (IPA) flavors in Kenyan coffees. Centals and Colombian coffee are usually a cleaner cup, often because they use a washed process instead of a natural process. Old green coffee will eventually lose the sweet flavors and can taste flat or cardboardy, which ends up with an unbalanced cup where bitterness can shine through. 

2. Roast: most of the sweet flavors in your coffee or developed during the roasting process. a very lightly roasted coffee can sometimes taste sour but will often taste with certain varieties tea like and floral. Medium roasted coffees usually end up with a good balance of sweetness without going into the caramel flavors of a more developed a roast. As you continue to roast the coffee you can develop caramel flavors and hazelnut flavors and eventually you'll start getting coffee that tastes more roasty, ashy, which will give you more bitter notes in the cup. Eventually you can get just burnt flavors out of a very dark roast. Caffeine is naturally bitter and chlorogenic acids developed in the roast are also bitter.

3. Days from roast:  coffee can improve in flavor for days and sometimes a week after roasting but starts to stale and after two weeks to a month will just start to taste stale and flat. Stale grocery store coffee often all tastes the same because the unique volitile flavors developed during the roasting process have left the coffee. Not much left of coffee but bitterness when it gets really stale.

4. Grind: the better the grind the sweeter the cup. Too many fines and boulders and your ground coffee can lead to both bitter and sour flavors.

5. Brew Temp: lower brew temperatures can lead to sour flavors and higher brew temperatures can lead to bitter flavors. This is especially noticeable in espresso. Between the extremes, lower temperatures can result in sweeter and more floral flavors and higher temperatures more caramel flavors.

6. Time since brewing: when you brew a pot of coffee the time that the coffee sits in the pot can increase the bitterness as chlorogenic acids break down and leave the bitter compounds. If you get your coffee from an air pot, you want to get a freshly brewed pot for the best cup.

7. Serving Temp: you can perceive the sweetness in a cup of coffee better as it cools, both for brewed coffee and espresso. If your coffee doesn't improve when it cools then it was probably a bad roast or brew. Dark roast doesn't improve much as it cools.

8. Cleanliness of equipment: unclean brewing equipment leads to dirty bitter flavors. A "seasoned" pot will taste strong and bitter. Your coffee shop should be flushing the espresso group on their machine and scrubbing the grounds off of there and cleaning the portafilter. seeing baked on milk on the steam wand is a bad sign for the cleanliness of equipment in a coffee shop. seeing oily stains in the grinder hopper is also one of the bad signs that I look for at a coffee shop. Shop sell mostly milk drinks, the milk balances out the bitter flavors of a bad cup so the important sign brewing and cleaning is minimized. People who enjoy a straight shot of espresso often only go to a select few shops that they know can prepare it properly, or brew their espresso at home.