A combination of (bot) IP addresses making 20+ requests a second, to post SPAM. Half the abuse was people just making lots of requests in short spaces of time, the other half was just people submitting spam, more slowly. I added rate-limiting to deal with the worst offenders, but mostly they'd keep POSTing away, ignoring the error-codes I was sending.

All submissions would get a random ID, something like this example chosen at random:

https://markdownshare.com/view/b5abadcc-c0fb-473e-a34d-f27c0...

There were numerous bots who'd try to spider/probe the site collecting all the links. Handling millions of 404s a month wasn't much of a pain, because the service was pretty efficient, but it was just another source of annoyance.

Finally, and God only knows how, I'd get a ton of DCMA takedown notices for alleged copyright infringement. That's what pushed me over the edge into first of all making it read only, and secondly planning to kill it at the end of the year.