All countries have opinions and information on other countries and groups that are best left unsaid. Regardless of global position or wealth this will remain true no matter what. Wikileaks takes these opinions and makes them public, and while everyone and his mother knows that these things are being spoken about, bringing them into the public view is completely unnecessary and will end up causing unneeded tensions in an already too tense global diplomacy.
WikiLeaks will not hurt the US diplomacy. The world leaders will be embarrassed by something they may have done or said, but that does not mean it will hurt their diplomacy. The public will just learn knew things they may not have otherwise been able to figure out. It could be a good thing.
If anything it would improve the integrity of US diplomacy. They will know they can't get away with certain things because wikileaks will call them out on it. People have said wikileaks has resulted in Americans deaths but I have yet to see a shred of evidence to support that claim.
The things WikiLeaks gets its hands on tend to not be a secret to diplomatic players the United States has to deal with. While it may tarnish the opinion citizens here and abroad have of our government, I highly doubt it'll ever do anything of significance to turn those that ultimately make the decisions against us.
World leaders know that their words can come back to bite them. They may be embarrassed when something that was not meant for public consumption is revealed, but they also know that the U.S. is too important a country for them to have as an enemy. Because of the power of the U.S., other countries won't let embarrassing leaks destroy the relationship they have with them.
WikiLeaks has the potential to hurt US diplomacy, but I do not think it actually will. Not everyone believes what they hear through WikiLeaks, or even bothers to look into it. Some people just shrug it off, or do not care see what other people are talking about. Right now, no, it will not hurt it.