I would agree to in general avoid being negative. And definitely try to shift conversations or questions towards your strength. Showing enthusiasm also helps, as well as attempting to demonstrate that you have a solid work ethic, are going to behave ethically and as a good colleague, things like that. It's amazing how often some people seem to focus on "getting the right answer" in interviews for school, residency, and jobs when the whole purpose of the question is often just to see how you think and respond.
If someone told me, "well gee I don't have any weaknesses at all!" I would be a little perturbed and probably a bit disturbed. You can answer it by talking about a specific area of pathology that you are weak in, I suppose, with the addition that you are trying to work on that area in your remaining training or conferences. But in most groups of enough pathologists, you are likely not going to have to do everything, so by admitting a weak area you might be able to get out of doing it 😉
When they say, "never say anything negative" it often refers to not saying negative things about your past employer, past training program, other colleagues, attendings, etc. Because that makes you look petty. To me it demonstrates a healthy sense of self-awareness to acknowledge our own weaknesses. Maybe it's just that you talk too much. Or you tend to be too analytical. You don't want to admit things that are going to be true problems, however, like "I drink too much" or "I have a bad temper, and I take out my frequent frustrations by punching things" or "I tend to be too overconfident when I make diagnoses."
Basically, you "ace" an interview by establishing a connection with the person interviewing you. If you have a good sense of self-awareness and awareness of other people, you can often gauge the interviewer's personality style and what they want out of you in the first few minutes, and you can guide your answers that way. I remember when I interviewed one of the partners seemed really interesting in figuring out whether I would be always looking for a job where I could focus even more on my subspecialty, so I made sure to keep talking about general path. Another one was obsessed with financial issues so I made sure to talk about that more often. The thing is though, you definitely want to be yourself. If you pretend to be someone you are not and they hire you on that basis, the job is probably not going to be a good one for you.