In my years at work I have given feedback on many, many different people during their annual assessments (reviews). I've performed assessments myself and simply given feedback on someone I've worked with. If I had any one piece of advice for someone giving feedback on a person it would be this:
Give me an example
For every feedback item I try and give at least one concrete example.
A general statement such as, "He leads a team well, but could be better at giving out task assignments" is ok. But a better statement is, "He leads a team well, but could be better at giving out task assignments. When I was working with him on XYZ he gave a great picture of what was necessary for the client to consider success. At the same time I had to go to him and ask what I should be doing or give him suggestions on work I should do based on my understanding of the deliverable."
Even if I miss 2 things that the person should improve on, having one concrete example gives the person being reviewed an understanding of where it came from and often insight into himself and ways he can improve. It also could be a misunderstanding on my part, which helps him see it's not really a problem.
Obviously this is true outside the workplace.
Every time I talk to my friend Rob about a problem or something I've seen that he could improve on, his first question is "Really? what happened?" Without the concrete example of what went wrong or what went right, the feedback doesn't feel like it applies. So it becomes easier to shrug off as not "my" problem.
If you can't find an example, don't give the feedback.