I think an experience along these lines is pretty common for WoW transmog enthusiasts: "Fantastic, I finally got the Talon of the Phoenix! I'm gonna transmog it to... wait. I'm using an axe for raiding." Or maybe this one: "Yes! This is an amazing upgrade! ...Oh I can't transmog a two-handed weapon into a main and off-hand, so I have to redo my outfit." I use weapons as examples because they often have the most limited transmog options, despite the fact that Blizzard did lift a lot of their restrictions in patch 5.2. Polearms and staves can be transmogged into each other. One-handed maces, axes, and swords can be transmogged into each other. Two-handed maces, axes, and swords can be transmogged into each other. But two-handed axes, swords, and maces cannot be transmogged into staves or polearms. Fist weapons cannot be transmogged into one-handed swords, axes, or maces.
In one view, the limitations make sense. As a druid, I can use staves, polearms, and two-handed maces. I cannot use two-handed axes or swords. If there were no restrictions, I could transmog my healing staff into a sword and be a resto druid with a sword. That seems odd to me. Then again, I can be a resto druid with a staff transmogged to a polearm, but I cannot be a resto druid with a staff transmogged into a two-handed mace, even though I can equip both those weapons.