I wouldn't say they're the opposite of Markdown, but that's not really the point I was getting at anyway. The poster I was replying to was saying that two of his issues with using HTML to write documents were the verbosity and the escaping. Slim solves those two issues.
Ah, apologies, point taken. Note that most of that style of templating language include markdown filters as well - so you drop :markdown in, then just write a block of text, works really well.