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

https://e621.net/wiki/show/uploading_guidelines#quality

On the upload page, there is a link near the very top of the page to the uploading guidelines.

hslugs said:
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.

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

My 2 cents about this is that it's more a matter of there being too many words we're asking posters to read and understand before posting. howto:upload isn't awful, but Help: Avoid Posting is a lot and where I would guess most people stop.

Especially when a user is excited to upload their first post, I would guess that if they look at Help: Uploading Guidelines at all, they're basically skipping it as they would a EULA. Too much text, too dense, doesn't focus enough on basic, critical information in a readable format.

...and I'm guessing even fewer of those follow the link to, let alone read, Help: Tagging Checklist. Information overload has occurred, and they just want to post.

I guess what I'm suggesting is that if we wanted to take a look at the problem from that angle, I'd suggest overhauling the documentation we present to new posters to focus on the most common flubs and missed expectations.