Charms can be attached to everything but blue and purple items.

blue and purple items are all uniques, handcrafted (with some small stat variation coded in it seems). They all have unique models as well. Supposedly 800 different uniques.

The sets seem to be restricted to blue and purple items. Its basically like in Diablo 2 from what i've seen. Not sure how much of the set you need to complete to get bonuses. Possibly all of them.

Legend:
Grey - Broken.
White - Normal items.
Yellow - Magical.
Green - Rares (the highest tier of loot with randomly generated attributes).
Blue - Epics (only uniques).
Purple - Legendaries/Quest items (only uniques). You couldn't have found another color for the quest items devs?
Orange - Charms/Relics.