You may be thinking that there are many illustration sharing services out there already. Of course, there are services where you can view contributed illustrations by category. However, doesn't it bother you when the category you want isn't available or the time it takes between submitting an illustration and the illustration showing up on the site is too long?

The main feature of pixiv is that users can tag illustrations they submit or view. This way you can quickly pinpoint illustrations which you have an interest in.
Tagging an illustration with tags which clearly express the illustration makes the illustration easier to understand and search for. (You can add up to 10 tags per illustration.)

Aside from tags, you can change the illustration's privacy level, comments and other settings. Bookmark the illustrations that you like. Easily view other users bookmarks and enjoy finding an extensive variety of illustrations. Also, you can add users whose illustrations you like into your "favorites" and their new illustrations will appear on the top page.

What do other people think about your illustrations?
Have you ever wondered that?
However, finding out in an objective way is fairly difficult.

At pixiv, you can rate other people's work on a 10-point scale. Based on the rating and other factors, the ranking (daily and weekly) is generated and it is easy to see which illustrations are popular. With many users rating and bookmarking your illustration, having it appear in the ranking, don't you think you'll get a clear idea of how highly it's rated?