Sage elaborated on various mechanics that PvE group members will encounter, such as healing, experience bonuses, role-swapping, switching alliances, and armor usage. He said that not every instance will require a four-person group to have the holy trinity, although the tougher dungeons will demand this.
Skill synergy was a topic that Sage addressed at great length: "Synergies are those abilities which another player casts, they are useful by themselves, but take on new dimension if another player comes over and activates the synergy effect. They are much more about reacting to situations and using them at the right time. They aren't necessary, but they do provide a large advantage when used correctly and may be the difference between life and death in harder combats."
Head over to Tamriel Foundry for the full read!
[Thanks to Phanes for the tip!]