What is Focus and when do you acquire it? (Votes are updated again. Skype friends and stuff.)
You acquire the Inquisitor's Focus about a quarter of the way through the game, after you arrive at Skyhold. A giant sucking void which causes damage over time and can banish demons when used close to a portal.
Other Focus abilities are unlocked via the character's specialties trees, which are unlocked after level 15, I think. Each of those is different but very powerful. For instance, Artificer's
Hail of Arrows doubles the effect of any archery abilities for a limited period of time, which is really powerful in combination with Varric's usual range of moves; Assassin's
Cloak of Shadows makes the entire party invisible even when attacking for a short while, making them immune to most enemy attackers, but still vulnerable to High Dragons' breath attacks; and Tempest's
Thousand Cuts is a nuclear bomb (figuratively), allowing you to teleport around slicing all nearby enemies between 12 and 36 times for high damage.
All the Focus skills require an energy bar to be powered up during combat, but persists between engagements, so you can run around fighting smaller foes to charge up before heading off to hunt down the High Dragons. Focus generation can be by performing combo attacks with other party members, such as freezing a target with a Mage then shield bashing them with a Warrior to shatter them, and can be upgraded three times via Inquisition perks, which doubles and trebles the effectiveness of each ability, depending on how much charge you've built up.
I'd say there's more balance between classes in this game than the others. Each has a well defined role in battle all its own while none are significantly overpowered compared to the rest. Generally the specialisations fall into the old Defensive, Offensive or Support models, but each has perks and twists making them useful in their own ways.