What information we do have is tentative in and of itself.  The names and icons associated with many of the skills are merely placeholders at this moment, subject to change at any time.  Still, I figure there's enough info out there to warrant an article about it, and some areas that could be better explained. 

Unlike Diablo, a skill may be placed in either of the skill boxes displayed on the status bar.  Each skill is mapped to a mouse button, the right side box to the right button and the left side box to the left button.  This means that a normal attack doesn't have to be your only left button option anymore, you can set both buttons to special skills as long as you have the mana to fuel them both.

Although my knowledge of passive and active skills are limited, I think I can shed some light on the subject.  Some passive skills (as I see it) will drain mana whenever they are activated.  An example of this is the Amazon's Dodge skill, which sidesteps away from enemy attacks and thus reduces the chance that a hit will land.  Each time the Amazon dodges, an amount to mana will be drained from her reserve, but the Dodge is automatic in and of itself.

Another skill that is passive is the Sorceress's Fire Mastery skill.  Fire Mastery increases the effectiveness of the Sorceress's fire spells (i.e. Fireball, Meteor, and Immolation).  But as far as I know this skill does not draw upon the mana reserves of the Sorceress.  Perhaps it increases the mana cost of each fire spell to reflect the bonus, the details at this point become sort of vague.