To me, regifting is one of those things where when it's done well, you never notice; you only notice when someone botches it, like by telling you about it or giving you something you had given them previously or presenting as new something obviously used. I think a lot of regifting goes on that people never notice, because the regifter is discreet and organized about it.
Also, I think attitude is important--not necessarily in an etiquette sense, but more in shaping the opinion of a third party. If someone's like, "I've got to get a gift for X, but I really don't care about it, so I will give them this generic thing I had lying around the house that I don't even know or care if they'll like"--if that's the only thing you ever hear about regifting, I can totally understand seeing it in a negative light. To me, that's in the same category as going out to buy, brand-new, a generic gift for someone, without caring whether they really like it or not. Yes, you had to put slightly more effort into it because you went out and spent money, but you certainly didn't put anymore thought or goodwill into it.
But I think it's also possible to regift with the attitude of, "This thing I happen to already have, which I don't really care for and haven't used, would be perfect for X! I would probably have bought it for X brand-new if I'd seen it in a store and didn't already have it." Give me that any day over someone handing over an obligatory, thoughtless gift.