#1 - Sept. 11, 2013, 11:20 p.m.
The idea of not having to sacrifice your character's visual when changing a piece of gear to a new one feels good. Since all rare items are fully random, including their visuals, you always have the chance of getting an awesome godly piece of armor with a crap visual making your char look like a murloc.
However, legendary items are different. They have an unique set of stats and unique visuals. If you let players simply change a legendary item's appearence OR change a rare item's appearence into a legendary display, you will just ruin the legendary itens VISUAL IDENTITY.
Here is the point: I have a great skorn axe with my barbarian. Skorn have an awesome and unique look, so any player who sees my character in game, will know that I'm using a skorn wihout having to inspect me. Furthermore, with transmog on legendaries, I could just by a cheap version of skorn with 10k gold, unlock it's appearence, and then transmog my rare axe into it.
My suggestion: make only randomly generated items eligible for transmog AND only ramdomly generated visuals eligible for transmog.