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If you read my art reviews, you can see that I try to be as encouraging to artists as possible, even if a piece is not so great.
People get this general misconception that discouraging an artist somehow will make them want to improve. But if an artist doesn't want to work on art, because they are discouraged, how will they possibly get better? The idea is to be constructive, but motivate the artist to continue practicing. If you read Template's review on my latest Daily, they make a point that "it can be better" as opposed to saying "it sucks". This suggests that they see improvement with some effort. That's the type of stuff artists appreciate.
I mean, some artists--I think myself included--don't care about being discouraged, as they pursue art on a personal level. Nobody can tell them to stop. But consider that not every artist is as invested. Like picking up a hobby. If you JUST started playing soccer and you find you're bad at it, it's not hard to quit. If you've been practicing for years, there is much more invested. So try to encourage everyone, as you never know who it'll affect and how.
Even if you like a piece, tell people what you like about it. Knowing what's right is just as important as knowing what's wrong. The rating system makes it too easy to submit an "opinion" without really fleshing it out. Positive reviews never really identify the merits of a piece, and negative people just tend to bomb the rating instead of offering criticism. Both sides need to improve their critiquing skills.