Pony: A button to notify contacts you choose about an interesting FPP, or an AskMe they're qualified to answer. For example, one of my contacts could easily give me a heads-up when there's a post about geography, or I could notify Astro Zombie about a Minneapolis-related thread. No more "paging languagehat" answers to etymology questions. If it gets spammy/annoying for th recipient, they can opt out or send me a STFU MeMail.
I'm picturing something like a "notify a contact about this post" button after the post content, which would generate a dropdown box and an automated MeMail to the contact with a link to the post. If they're not your contact, you have to MeMail them the old-fashioned way.
Yes, I know I can look for geography posts on my own or subscribe to tags/categories via RSS, but if a thread derails into a discussion about a favorite topic, I'd love to know about it.