A lot gets said about evaluating technical skills when building teams, but I find that to be relatively straightforward - what's trickier is understanding how a person prefers to work and so how much effort (i.e. change) will be needed for the team to gel once that person is added. Consequently, I spend a lot more time thinking about this than any specific skill a person might have.
It's as if each of us has a personality blood-type. Just as transfusing blood among individuals without taking account of their body chemistry can result in serious health issues, so too mixing otherwise competent individuals without regard to group chemistry can greatly reduce everyone's effectiveness. This is true, even if these same individuals would be very effective in different groups.
Like one's actual blood type, there are multiple factors involved, but I find three especially important. So, to run with the metaphor let's call them 'A' factor, 'B' factor and 'Rh' factor:
- A Factor: Knowledge Sensitivity: A person has this trait if I expect the effort needed for them to understand the Situational Context of the team will be a significant factor in the gelling process. Considerations include: current knowledge, learning preferences (online research, training, books, etc), learning-speed, learning-skill, open-mindedness and how (and how easily) they'll learn from their teammates.
- B Factor: Environmental Sensitivity . This stuff matters to most everyone, but some are more sensitive to it than others. If it looks like it'll be a prominent part of the gelling process, they have this trait.
- AB: Person has both traits.
- O: Person has neither. I don't expect the person will require radical shifts in team cohesion to gel with the group (though they still may cause such shifts over time).
- Rh Factor (+ or -): Tendency toward taking social initiative. This one's hard to define. To get a sense of what I mean, the next time you're in a casual group discussion (ideally of peers), note the people who tend to pick the topics of conversations, suggest lunch destinations and so on. These people have this trait. When work topics arise, they're the ones proposing new ideas and things that need focusing on in the first place. Incidentally, if you believe you have this trait, you might need to reign it in to get a sense of who else has it (and a good habit for you to develop regardless).
I see these factors as neither strengths nor weaknesses. Nor as (necessarily) fixed traits. A successful team may contain (or require) people exhibiting any combination of these traits. Adding people who learn quickly or who care a lot about how customers are acting toward the group or are passionate about their work environment  is just as important as having deeply experienced people who are agnostic to the details of the work environment. Even when people's quirks or lack of knowledge might seem to be burdensome, our work is a creative activity not a mechanical one. Taking the effort to incorporate people who have a particular genius or ability into my teams has paid off more often than it's been a liability.
It is also important to understand this is my thinking process after it becomes clear the person is a plausible fit for the team. That means they've already passed the no assholes rule, otherwise appear to have the skills the group needs and I believe will get along reasonably well with the rest of the team outside the context of the goal at hand.
Of course, I also consider many other non-skill traits to be important - e.g.: risk aversion; time flexibility; tolerance for ambiguity, tool/practice prejudices and social neediness are all relevant - but regardless of which ones dominate a particular gelling process, what I'm always after is a better understanding of how the person will complement the rest of the group. Most importantly, reflecting on why people work well together helps me to do my job better. Hope you find it useful for that too.
 More precisely: How sensitive the individual is to forces acting within or upon the group; this important topic will get a separate blog entry hopefully soon
 Leadership in the narrow sense of desire to influence direction of a group of people.
 Tool and process preferences is an entire - large - subcategory that can even be considered an entire trait all it's own. We do so love our tool and practice religions!