What items can be used in transmogrification?

Blizzard may change how this operates yet but here is the link to their page saying how it will work :-
http://us.battle.net/wow/en/blog/3309048

Important points

• So you can only use items that are made for your class, so if you wear cloth, you can only use cloth items etc.
• The items must have stats on them.  Green items and higher have stats, you will not be able to use white items.
• They appear to be still discussing whether to use special items. e.g. items obtained at holiday times.
• From what I have gathered, you will go to an NPC like you do when you reforge an item and will be able to preview what the tranmogrification will look like.  You will also be able to undo a transmogrification if you want to.
• Until this patch is released, the way this operates and the items you can use may change.  I will do my best to ensure my information is correct but there may be items I use that Blizzard do not allow when this goes live.