There seems to be several distinct kinds of C&C.
There's pros and cons with almost all of them.

1) 'Just wanted to say I liked your story, man.'

PROS: it's encouragement.  Us fanfic writers thrive on
      encouragement.   It's like sunshine to houseplants.
CONS: gives no idea as to WHAT the heck was liked, or any
      advice on how to make it better.

2) 'You made the following typos:...'

PROS: hey, it's always good to repair some glitches like this.
CONS: leaves the author feeling insecure.  He just got mailed
      an e-mail telling him everything he did wrong, but there
      was no feedback at all on what was being done right.  Leaves
      a guy feeling... well... insecure, ya know? n.n;

3) 'Your fic was good, but there were a few problems here and there...'

PROS: Gives the author an idea of what the heck the readership wants
      to see, which is a good thing.  Right?
CONS: If you're writing to an oversensitive freak, then you just might
      have made him quit, thus killing a fanfic that you saw some
      promise in.

4) 'You fic sucks!'

PROS: well, a fic that you detest just might get squashed.
CONS: who knows, maybe the next fic the guy writes will be better?
      for all you know, you just crushed the hopes of someone
      that might be the next William Shakespeare.  Or maybe not.


ON WHAT THE WRITER WANTS:

NOBODY wants to hear #4.
The C&C that dreams are made of are made of #1-3
The very least one can ask is for #1
Getting #2 can almost be as depressing as #4, so please, even
  if it's short, do #1 AND #2.

And most importantly, people, support your favorite fanfic writers.  Some
quit just because of the silence, and that's a shame.

-rpm, 
 going to find the Rush Limbaugh 'Evil Japanese Tentacles' article...