(For the record, I'm totally making these examples up).
Frank: Really smart, but has trouble focusing on work.
Melvin: Can't explain things well, but is a wizard with the highly intermittent, hard-to-track-down problems.
Claire: All around good, but not as fast as she thinks she is.
When I ask people to do this, they'll generally come up with a good and bad balance. "Bad, but good", or "Good, good, but bad". This applies to either individuals or entire teams. If you have a situation where you can't think of a good thing about a person, or if you can't think of a bad thing about a person, then there's a problem. People and teams are, well, human. There are always good things and bad things about their work. If you can't see both sides, ask yourself if you really have someone who is that extreme. More likely, you're biased in some way, for or against. It's these types of biases that can blunt our effectiveness as managers - we're not looking at the whole person or team. And yet we do summations all the time, explicitly (during review season, for example), and implicitly (when deciding to whom a task should go).
Overcoming the snap judgements about someone's effectiveness is hard, but most of it comes down to committing yourself to a balance. For someone you think is all bad, find something that they do really effectively. For someone who can do no wrong, look closely and don't stop until you find something that they're not as good at (that mythical perfect team doesn't actually exist, unfortunately). Use a written pros and cons sheet, and remind yourself of the whole picture when you need to come to judgement on someone or some team.
Be mindful of how you're considering people and teams, and make sure that you're not hurting yourself and the team by letting yourself see only one side. There are two sides to every person, and to every team. You owe it to them to look at the whole picture.