They've promised that their censor bots will learn and get better.

The mistakes have been pretty amusing. One person had nude pics up; it left those, and flagged an image of her fully clothed. I had a photo banned that was just someone's face. Not even the neck was showing, let alone any naughty bits. Others have had silly cat pics and photos of dishware banned.

I've found they're pretty quick to reinstate images mistakenly flagged. Just click the "review" button. It was 12 hours for the first image I challenged, and about 2 minutes for the second one.

This has actually been planned for a long time, and was probably inevitable as soon as it was sold to Yahoo/Verizon. Though the app being removed from the Apple store due to child porn issues affected the timing, it was coming anyway.

Basically, it comes down to money. Tumblr was hemorrhaging money. They need to "monetize" it if it's going to survive. And that means selling ads. They can't sell ads (at least, not the kind they need to sell) if they'll appear next to adult material.