This month on NaBloPoMo the theme is "letters". That can mean correspondence, of course, and my guess is that this is the inspiration most participants will follow. But for the typography nerd, letters can mean only one thing. I'm beginning with my favourite letter of all, which also happens to be the "first" letter on the QWERTY keyboard.
I love its round form, combined with the flourishing possibilities of the tail. Lower case can be fun too, especially in any hand based on 16th-century scripts, like this one:
For me the letter Q offers a quick way to decide whether I like a typeface or not. Because almost without fail if the Q is gracefully formed and perfectly balanced, then I'll like the other letters. And if the Q isn't right then it doesn't bode well.
Take the font you're reading now (Georgia) – I'm not entirely fond of the way the tail of the Q descends from the middle of the main letter. But there isn't a lot to choose from in the TypePad set and Georgia wins because it also has old style numerals, which sit within the x-height (1, 2) and have descenders and ascenders (5, 6).
But the possibilities for Q are endless, as this tiny sample reveals:
My preferences are mainly clustered at the top and left; at far right is one of my least favourite Qs of all: Times New Roman. The disappointing Q just above it is Bodoni, which is actually a font that I like very much. Bodoni would be so much better if it had a Q like Modern No.20 (the Q at bottom left). The lowest Q of all (Bell MT) should in theory fall in the favoured group, but the balance is wrong, with tail weighted too much to the left; just above it to the right is another that I'd like a lot, except it looks like the tail has been thrown at it and just missed (it's Calisto MT). The large watermark Q is Monotype Corsiva – very elegant. And among more formal faces I like Baskerville's (top left).
There's another reason why I'm fond of Q. 
It's the letter behind the question mark, beginning with "q" as a shorthand for the Latin word for question (quaere).