1) We've seen some of the particle effects in screenshots, but what sort of effects can we expect otherwise? Elaborate, flashy displays? Geometries and patterns fitted to the spell? Intensity based on the level of the spell or spellcaster?

You haven't seen any of our real particle effects yet; those were just placeholder particles. We intend to provide a mix of subtle and elaborate spell visuals and casting animations. We have a dedicated particle team that is putting together some incredible stuff.