Something that's in the middle between a bug and a missing feature (imho) is that Slack's failure to support Github-style markdown.

GitHub does a great job with markdown, especially with programming-language-specific highlighting. And even things like strike-though text and URL's, which aren't specific to software development.

It's one of the few aspects of Slack that has me open to alternative services.

that's a bit inflammatory to say; especially without offering a single example.