I found a circle all to myself after the servers came up yesterday - here are some of the invasion mechanics I discerned.

Two types of mobs drop epics - the Shades in the middle, and a variety of Rare spawns. So far, i've seen 3 rare spawn models - silvery phantom (can visually blend in with the regular ghosts, so watch for these in particular), skeletal mage, and zombie glutton/patchwerk type. The rare spawns seem to have a 100% chance to drop epic legs. The shades seem to always drop a stack of 30 runes, and have about a 50% chance of dropping an epic chest.

If you summon a Shade, then wait a while, another conjurer will respawn in his place. This won't happen indefinitely, but I did get 6 shades out of one circle. I was really hoping for more rare spawns, and occasionally killed one of shade while waiting. Eventually, the circle faded, and the regular mobs all despawned over the course of a few minutes. The conjurers didnt' despawn, but I Shade-ed them all pretty quickly once I saw the circle was gone. I would guess you could kill 3 Shades, let them repop, kill 3 more, and let them repop again before the circle dissapeared, if the circle disappears on a timer.

The regular mobs around the circle spawn AND DESPAWN regularly. I have no idea why, but the practical meaning of this is that rares will likely despawn if you take your sweet time getting to them (which I wouldn't do anyway, since someone else may happen by and tag them).

There's one armor set each for plate, mail, leather and cloth. Each is dps oriented, only the cloth is caster oriented, and you cannot mix and match the pieces for the set bonus (i.e., mail and leather have different set names, though the bonus is the same).