Since I wasn't at my home PC when I posted this thread, I couldn't add my own transmogs. I theme it by spec: I use our Challenge Mode set when I'm Arcane (still need to find a staff to go with it), and the below two as Frost and Fire.
My Frost set is themed around the weapon, the Frostscythe of Lord Ahune. My only requirement was that the set match in colour and use a robe. It's almost all from Ulduar.
Head: Lifespark Visage
Shoulders: Iceshear Mantle (Underworld Mantle would also work)
Cloak: Deathchill Cloak
Chest: Raiments of the Corrupted
Bracers: Combustion Bracers
Weapon: Frostscythe of Lord Ahune (Greatstaff of the Nexus and Shaft of Glacial Ice also match pretty well)
Gloves: Static Charge Handwraps
Belt: Sash of Ancient Power
Legs: Not 'mogged, as they're hidden beneath the robe
Feet: Velvet Boots of the Guardian
My Fire set is our tier 5, which I've always loved for its simplicity. Getting the right weapon to match it was a struggle, but I was able to loot the limited-time Archmage's Staff from the Theramore scenario at the launch of MoP.
Head: Cowl of Tirisfal
Shoulders: Mantle of Tirisfal
Cloak: Bloodmoon Cloak
Chest: Robes of Tirisfal
Shirt (because there's a gap between the gloves and shoulders): Gorgeous Blouse
Bracers: Combustion Bracers
Weapon: Archmage's Staff
Gloves: Gloves of Tirisfal
Belt: Fire-Cord of the Mages
Legs: Not 'mogged, as they're hidden beneath the robe
Feet: Velvet Boots of the Guardian