This one should be common knowledge and in a way I feel like I'm stating the obvious. But then I'm also aware that many designers still don't do this, so for what it's worth, here goes...
You see, whether we like it or not, most designers are accidental typesetters. Few of us ever set out to be typesetters, but we find ourselves in a time when as part of our daily work we're knocking out text on our Macs. It can be a big part of our day.
Problem is, lots of us are still doing just that, knocking it out. What do we know about the details of good typography/typesetting? In the old days (ah, the old days...cow gum for goal posts...) we had typesetters to take care of the details for us, details like this:
Of course, if you're from the US you might have a liking for the em-rule, but that's another story.
This kind of stuff reminds me of the Fletcher quote, "You can only muck about with language if you know what you're doing. Otherwise you're just being sloppy". I think the same goes for typography.