I remember reading a particular long story on the archive, one which touched my heart and gave me goosebumps. So I e-mailed the author and gushed over the story, and she responded with surprise and not a little humility. She was quite gracious, and I've enjoyed her stories ever since then.
And I must confess, I don't leave enough feedback for WIPs either. Sometimes I see that another person has mentioned the same things I have, or there's another story I want to catch up on, or I have to go check me own e-mail to see if I've gotten any response from my Beta -
You know how life goes. Sometimes it's even something from RL that keeps me from the boards, like plumbing contractors or financial pressures or my daughter's on MySpace.
I hereby resolve to give feedback to writers whose WIPs I read and enjoy on these boards. I also resolve to e-mail authors whose works I read from the archive, assuming I can find something mildly original and complimentary to say. It's one thing for people to tell me they don't like the direction of a WIP, but I don't know that anyone likes having a completed work slammed.