The last blind tasting for this experiment was done last night. Here are the results along with a conclusion for the experiment.
All tasting of the beer was done blind. It turns out that half of the people actually preferred the tap water beer over the distilled water beer. As I stated in an earlier thread, SWMBO was one of those, but she couldn't describe why. She tends to like more bitter beers, so maybe the higher sulfate content of the tap water beer accentuated the bitterness. The other person that liked the tap water beer described it as smoother and noticed a slightly darker color. That person described the distilled water beer as not having an aftertaste, less body, and sharp. They thought they detected a bit of chlorine, but they weren't certain.
I along with another one of the blind tasters preferred the distilled water beer. I definitely detected the harsh quality in the tap water beer that inspired me to do this experiment. To me, the tap water beer was sharp, harsh, and biting. The distilled water beer was smooth and not harsh at all. The other taster who preferred the distilled water beer described it as light, low carbonation, and no aftertaste. They described the tap water beer as slightly bitter in the back of the throat (aftertaste).
All brewers must be cautious of the sodium and sulfate levels in their brewing water. The extract brewer has an extra burden because they also inherit the water profile that the extract was made with.
High levels of sodium and sulfate (in my case over the 200ppm level) can lead to a beer that is harsh and astringent to some people. Others may actually prefer a beer made with this water. I am convinced that the harshness in my batch that inspired this experiment was due to high levels of sodium and sulfate.
Using all distilled water may not be the best answer either. It may lack minerals and the final pH may be off.
If you notice that your beer has a harsh aftertaste, and you are an extract brewer, try brewing with all distilled or RO water. This will solve the harshness problem, but you may need to tweak the water to get the proper pH.