In the list view in What's New, add a text entry field under every submission to enter a comment without having to click through to the submission. Feedback and approval are a huge factor in what keeps artists motivated to continue posting, and if you remove a click from the process, you'll encourage more people to do it.
Additionally, it would be really nice if there was any kind of incentive for people to leave feedback on artwork, even if it was purely cosmetic, like achievements (i.e. profile badges for 10 comments in an hour or 100 in a week, leaving a comment on every image in an artist's gallery, X comments in excess of 50 words, etc) or a karma rating in relation to number/length of comments left in the last four to six weeks.
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