Do state somewhere explicitly that submissions are being vetted for quality, artistic and otherwise. Preferably somewhere on the upload page.

New users probably expect this "approval" thing to be a simple spam filter to weed out real-life porn and image macros and just unrelated stuff. When their uploads clearly aren't any of that, and still don't get approval, they don't understand what's going on and come complaining.

CCoyote said:

• not understanding that e621 is a curated archive and not a gallery

New users have no easy way of learning this.
The "curated" part is not obvious and lots of people probably do not expect it.