I've done the area up to the Regrowth twice (and the entire area once), and I haven't seen any difference in letting the harpy live or killing her. When my hunter let her live and did the entire zone, nothing about her ever came up again. As for my mage, It's possible that killing the harpy would come back to bite me in the ass later, but I can't say for sure. Seeing as how I don't remember any of the post-Regrowth quests having anything to do with the harpies, I don't think the decision has any consequences.

And now that I think about it, I'm a little disappointed. Blizzard gave us what looked like a choice in a quest - and a moral one, at that - and didn't follow up on it. It would have been nice to have the decision acknowledged elsewhere, at the least.

Now, if it comes up again later in the 4.2 daily quests, all will be forgiven. ^_^

I think it's Blizzard's way of starting to incorporate a good/evil meter like in BioWare's games.

It was more that it, and a couple of other places like it, were a response to peoples reaction to the torture quest in Borean Tundra than an actual moral choice, because some people didn't like the idea of cold bloodedly torturing a dude.

So perhaps people are expecting a little too much here.

The torture quest was supposed to make you feel uncomfortable.

The one time I set the harpy free she just attacked me again anyway and I killed her. I guess it's the thought that counts.