It is interesting in the case of coffee that the right amount of hardness is critical for getting the best possible brew.
I was at a lecture on the chemistry of coffee making (sponsored by a maker of commercial barista coffee machines) and with an expert present with all the methods of making the stuff and tasting some very exotic single estate coffees and single species/varieties.
The most fun was him making coffee with different qualities of water:
- Pure water (reverse osmosis potable grade)
- Newcastle tap water
- Evian water
It was recommended to taste them in that order too.
#1 was OK but rather soft, dull and lacklustre. #2 was a very nice balanced cup of coffee. #3 was disgusting with free bases that tasted really bitter.
Turns out Newcastle tapwater is almost perfect for coffee making.