The exclamation mark is used in the English language to signify a situation of heightened emotion, for example in a warning message, or a shout of anger or other strong feeling. It is generally used quite sparingly, in fact it is possible to read entire novels and not come across one. This is because novelists are generally literate enough to realise that over-use rapidly diminishes its impact, and that context and the meaning of the words themselves are normally enough to communicate the state of excitement.
The following would seem to me to be examples of valid usage:-
All three cases decribe situations where a genuine emotional response would be expected, hence the exclamation mark seems natural.
But, as I have noted, the exclamation point is rarely seen, except in two types of medium. The first is the kind of shrill advertising that shrieks: 'Sale!!! Save £££ on a cheap and nasty sofa!!! The second is as used by programmers. I found the following examples recently in a newsletter extension for Joomla!**:-
Notice that in these examples there is not just one but three exclamations. Presumably if one is intended to convey excitement, then presumably three should convey three times that amount. Or should it be the original amount of excitement cubed? Who knows?
The first example quoted above occurs when a user successfully activates their newsletter membership, the second when they have forgotten to include their email address. I am not going to name the guilty programmers responsible for this, as I have seen this kind of usage quite widely. In fact I have occasionally caught myself doing this. Creating user accounts, resetting passwords or inputting incorrect user information are routinely met with a whole string of exclamation marks.
I can honestly say that the most emotion I have ever felt when successfully registering with a website is quiet satisfaction, usually it is not something I feel strongly about either way. I would guess the same is true for most people, except perhaps the chronically over-excited. It makes no sense to meet such mundane activities with three exclamation points. To me, this kind of usage is a sign that the programmer is not really thinking about what is being said to the user. It comes across as merely shrill, rather than generating any real emotion from the user, so is quite ineffective.
The poor exclamation mark is sadly overworked - so programmers please give it a rest.
* Had it been "Oops! I dropped the plutonium!!!" I will concede that three exclamation marks might be appropriate.
** ironically I have to use the exclamation mark here, as it is part of the name 'Joomla!' Sadly I think it is too late to do anything about this.