Multiple exclamation marks
There are a few mentions of multiple exclamation marks in the books and by Terry Pratchett himself, so it appears this is something he feels fairly strongly about. The basic idea is that a person's sanity is inversely proportional to the number of exclamation marks they use!
There is a practical demonstration of this in the final act of the Opera in Maskerade, when Salzella is unmasked as the malevolent opera ghost. (Corgi PB, pp 348 - 358). At first rational, explaining his actions and motivations and looking for a way to escape, there are no exclamation marks at all. He behaves in a perfectly rational way for an unmasked criminal, taking Agnes hostage and using her as a bargaining tool to secure his freedom along with the stolen money. He can even explain why he really, really, hates opera as an art-form while resorting to no exclamation marks whatsoever. But there is no escaping the fact that the final act is being played out, onstage, at the Opera House, and single exclamation marks begin to creep in the moment Walter Plinge, as the good ghost, appears on stage to confront him. From the moment he says "Oh, yes! A ghost of a Ghost! Totally unbelievable and against all common sense, in the best operatic tradition!" he is doomed to an inexorable slide into insanity.
"You don't know what it has been like, I assure you, being the only sane man in this madhouse!!"
Temporarily gaining the advantage over Walter/The Ghost in a stage swordfight, as Walter trips over Christine's unconscious body, he goes up a further gear into madness:
"See!!! That's what comes of believing in opera!!!"
Thinking to end things by humiliating Walter back into idiocy, he then goes one further:
"Really, Walter!!! You are a bad boy!!!!" and "Look how everyone's staring!!!!"
This is serious stuff: earlier in the book, nobody (other than the Ghost, and then only in writing) has gone any further than three exclamation marks. For the Opera, this is new territory.
At this point, Granny Weatherwax steps in and calculatedly pushes Salzella to the full five exclamation marks, which turn out to be critical mass, as in a renewed stage duel he believes in the reality of what he is doing so much that he dies, even though the killing sword blow has only passed harmlessly between his torso and arm.
"What gets me is the way that in opera everybody takes such a long!!!!!......time!!!!! to!!!!!...... argh...argh...argh..."
Shortly after this, Seldom Bucket goes up to three as he gets more frantic concerning where the stolen money is, but is talked down by the Lancre witches. However, the excitement involved in contemplating the large sums of money to be earned from staging musicals rather than high opera drives Bucket into a different sort of five-exclamation-mark insanity on page 364...
Quotes
'Multiple exclamation marks,' he went on, shaking his head, 'are a sure sign of a diseased mind.' -- in Eric
Five exclamation marks, the sure sign of an insane mind. -- in Reaper Man
'And all those exclamation marks, you notice? Five? A sure sign of someone who wears his underpants on his head.' -- in Maskerade
CITY'S BIGGEST CAKE MIX-UP!! [...] 'Lose the second exclamation mark,' he said. 'Otherwise I think it's perfect.' -- William de Worde in The Truth
It goes 'baa!' It is a sheep! [...] That! Is!! Not!!! My!!!! COW!!!!! -- in Thud!
And, by Terry himself:
[ Terry's wife Lyn reads all his mail first, and selects the reply order ]
"It tends to arrive on my desk in this order:
- Stuff that really needs to be dealt with today.
- Stuff that needs an answer quickly.
- Fan mail with SAEs (Lyn encourages politeness) or which is particularly interesting, worthy, funny or whatever.
- Any other mail from abroad (because it's usually taken a while to get here).
- Other mail.
- People who send me their MS without checking first, and others of that kidney.
However, I tend to stir it all up and in fact answer in the order:
- From kids
- Typed
- Readable
- Interesting
- Others
- Ones written in green ink on mauve paper
- Ones with more exclamation marks than sanity dictates"