My main gripe is people who use hyphens as en-dashes in display type - it looks awful; and these people chrage a fortune for their supposed design ability.

Another gripe is em dashes used with spaces on either side. Only en-dashes do that.

As a general rule for parenthetical use, British form is 'space en-dash space' and US form is just an em-dash with no spaces.

In screen work, I use a single hyphen over a double hyphen -- for a dash. The double thing is a hangover from typewriters, along with double spaces after a period.

I really pay far too much attention to all this sort of thing, but it is a good way of telling if someone knows what they are doing; which is useful as I have a sideline in providing critical appraisal of design work to companies who've just paid some London agency 50 grand to redesign their business cards.