Right now, I'm relying on Akismet [1] to automatically filter spam, which is a
really good service. It's built by Automattic and they've had years of spam data to work with.
I haven't thought about forum slides, but that's pretty interesting too. I don't think Akismet could detect something like that. Of course, you can do it with moderators (you can make it so that all comments must be approved; or you can auto-approve and delete the offtopic ones later), but an automated way to detect that would be cool.
This is the long-term research plan I have; maybe we can even do away with Akismet. Spam is one of the hardest problems to solve, so it'll be interesting. But for now, I'm relying on Akismet until there are fewer and fewer core features left to implement.
Thanks!
[1] https://akismet.com/