alt Hacker NewsThat's exactly what we used in on usenet (except,without rendering unless you were using a nice GUI reader, not just tin/rtin)
The problem is that that's too many characters to reserve (they all have to be escaped when you want the actual character) making the resulting text look awful in plain text mode.
They are not reserved characters. They only express a special format when used in that way : a space on its left, and a character stuck to its right AND somewhere down the road : its twin with a character stuck to its left and a space on its right. I built an editor doing just that more than two decades ago, and it works fine.
So *this* and /that/ express formatting, but not 4 * 5 nor 4*5 nor 4/5 nor m/s.