Unfortunately not everything can be transmogrified, putting to rest my dreams of PvPing with a giant fish weapon channeling Michael Palin. The following is the list of Transmogrification rules as defined in September 2011. Please remember that this was before the launch of Patch 4.3 and the rules are expected to change somewhat as they analyse practical use of the system.

What follows is our current list of rules for transmogrification. This is a set of rules that we expect to change over time, and is not intended to be 100% comprehensive, but we hope that it can be referenced to answer most questions that players currently have about what they can and cannot expect to be able to do with transmogrification.

• The character must be able to equip both items.
• Only uncommon (green), rare (blue) or epic (purple) items may be transmogrified. (exceptions: a select few of these items will be prevented from being used to transmogrify if they are inappropriate)
• Items must share the same armor type (examples: plate for plate, cloth for cloth)
• Weapons must be the same weapon type (exceptions: Guns, Crossbows, or Bows)
• Guns, Crossbows, and Bows can be used to transmogrify Guns, Crossbows, or Bows.
• Main hand weapons can only be used to transmogrify Main hand weapons.
• Off-hand weapons can only be used to transmogrify Off-hand weapons.
• One handed weapons can be used to transmogrify a Main hand or Off-hand weapon.
• Using an item for transmogrify makes it soulbound.
• Using an item for transmogrify makes it non-refundable.
• Using an item for transmogrify makes it non-tradable.
• Heirlooms and Account Bound items can be transmogrified.
• Heirlooms and Account Bound items can be used to transmogrify.
• Legendary items cannot be transmogrified.
• Legendary items cannot be used to transmogrify.
• Fishing Poles cannot be transmogrified.
• Fishing Poles cannot be used to transmogrify.
• Mailing an item strips its transmogrification.
• Placing an item in Void Storage strips its transmogrification.
• Vendoring an item strips its transmogrification.
• The displayed enchant will be that of the currently equipped item.

There may be individual items that are excluded from being transmogrified on the basis that they were originally added to the game as absurdities. (examples: a weapon that looks like a fish, or a chest piece that is invisible)

http://us.battle.net/wow/en/forum/topic/3196342583