<sarcasm>Throws toys out the pram</sarcasm>
For aaaggees now I've wanted a way to express my sarcasm over the internets. When you're typing this can be pretty hard... it can get lost pretty quickly when words on a screen don't come with tone.
Enter the <sarcasm> element*. I, (as well as others), have been know to add this to IRC chats, or at least end a line with </sarcasm>.
It suddenly changes the tone of the conversation. You become enlightened as to body language that otherwise gets missed when not in direct vicinity with another person.
And why stop at sarcasm, could we express other emotions too - just with one element and it's attributes?
<emotion type="excited">I'm going to the fair tomorrow!</emotion>
Are there too many emotions, (and variations of such), or maybe I'm just about to step into a debate about how emotions don't matter and only logic prevails.
<emotion type="scared">I'd better get out of here!</emotion>
*This idea is in no way practical.
EDIT 9.1.15: There is a sarcasm punctuation mark, as I discovered whilst watching University Challenge the other day. There's more about it on the sarc mark website.