I learned a lot about management during my graduate studies - learning by example sometimes, but mostly learning from counter-example.  Being a manager frequently means giving feedback, and I've learned a number of lessons there. Over time it became obvious that there is one iron-clad rule that must be understood: If you do not give credit where credit is due, you have zero credibility in giving criticism where criticism is due.
Note that this is not the Golden Rule, this is not a piece of advice. This holds true regardless of what you think of it, and especially if you think it's not true for you. It's also not a matter of being fair: some of the most manipulative people I've ever known have cultivated a reputation for fairness.
Those who are oblivious to this rule are easy to spot: people don't listen to them. They're called nitpickers, tyrants, "never satisfied"s. These are the people whose children and employees are told privately, "You can't please him, so just please yourself." Working for someone like that can be an exercise in frustration, and it rarely ends well.
If you are a person who does not understand this rule, you are probably saying to yourself, "It shouldn't matter if I say positive things so long as what I am saying is true." In theory, that's true. And in theory, there's no difference between theory and practice. But this is not just a matter of discernment and truth, it's a matter of trust. Very few pieces of work that you will ever be presented with are entirely without merit, and if you cannot be counted on to identify what is of merit, how can you be trusted to have identified correctly what was wrong?
This is not a matter of "softening the blow." This is not a matter of trying to find something nice to say whenever saying something negative. It is a matter of cultivating a reputation for fairness: if the person you are talking to understands that you are the kind of person who will identify and comment on the positive aspects of their work or argument, then they will listen to what you are saying, even if this particular interaction is wholly negative. In fact, earning and maintaining a reputation for fairness is remarkably freeing. It is the only way to legitimately be able to say what you think.
This leads into a corollary - you must understand this rule in order to correctly apply it. If you do not understand it, and think of it merely as "softening the blow" or (worse) as "being nice" then you will inevitably wind up spouting mealy-mouthed crap. You'll wind up focusing on trivialities on the positive side, and only getting to the meat on the negative side, and anyone with half a brain will consider this at best a nicety and at worst condescending. If you are presented with something that you wish to critique, you really honestly cannot identify anything non-trivially positive in it, and have not earned a reputation for fairness, you would be far better off keeping your mouth shut: you will probably not be heeded, and are probably wrong about there being nothing non-trivially positive.
I bring this up because I see everywhere people who violate this rule constantly, and then wonder why nobody takes them seriously. Politicians stand up and rail against the new President for what they see as waste, for example - and you know what? There is a lot of waste in both the stimulus bill and other spending bills. But they have no credibility to criticize, because they fail to recognize what is being done right, and they additionally failed to point out the same offenses when they were committed by friends. Without credibility, they have nothing.
There is in general on the Internet a lack of understanding of credibility. I could point out a number of online discussions and online "discussions" where the people talking seem to all assume, "Being right justifies what I say, how I say it, and how I treat those who disagree with me." Put an otherwise normal human being onto a message board with a tiny comment window and a lot of semi-anonymous handles, and that person turns into a college freshman, or worse. I am coming to believe that this is a perhaps necessary result of valuing anonymity - one CANNOT have credibility under these conditions. Anonymity encourages the idea that every comment is an island, but most forums discourage posting long enough comments to engender enough credibility to make an argument - even if the comment boxes are large enough, and the comment screen doesn't cut off or otherwise hide long comments, you simply lose track of a "real-time" comment thread and risk having someone faster or more pithy than you make the case while you're still checking your spelling.
 I have a whole other rant ready to go about how PhDs are not adequately trained in management, but almost universally expected to manage. Having gone through the experience of getting a PhD is not a guarantee of knowing how to manage other people - it is if anything a hurdle to overcome when learning to manage! Too many PhDs get into the "I'll do it myself" micromanaging mode, or insist that everyone have a special portion of the project carved out for them as though they are working on theses and not together on a shared goal.