team composition
While there are several elements a team needs, such as varied capabilities (at least one ranged attacker and one damage soaker), the more important elements answer the question "What do I bring to the team?". Any team needs people who are good with a battle plan, those that are good at directing people, those that can keep the team healthy, and finally and possibly most important, those that are good at deciphering clues. With these elements, a decent team could even be developed from a single archtype.