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.
Honestly, I'm getting a little confused just thinking about it. Taken without any given, specific class's equipment restrictions, the weapon transmog rules seem pretty straightforward. Staves and polearms seem to be more similar in spirit than swords or axes. However, once I start really thinking about the way it affects individual characters because of the overlaps in equipment, it gets more bizarre. Maybe a better rule would be that you can transmog any two-handed weapon into any other two-handed weapon that you can equip, and you can transmog any one-handed weapon into any other one-handed weapon you can equip.
Then we have armor. This is a little less of a hassle. Again, as a druid, I can pretty much transmog any leather chestpiece into any other leather chestpiece, or shoulders into shoulders, etc. It does make me sad that I can't transmog across other equippable armors. In TBC, I used the Robes of Heavenly Purpose for a long time, and I loved them. It's a gorgeous model that looked fabulous on my female night elf. I still have them in my bank, but the robes are cloth, and I can't transmog my modern leather gear into cloth. Then again, if all restrictions were lifted, wouldn't it be odd to see warriors running around transmogged into a priest's outfit? Or vice versa? That could be interesting, especially in PvP!
Overall I will admit that I find the transmog restrictions to be frustrating at times. However, there is one reason why I am secretly kind of grateful for them, and that is because my bank and void storage are already stuffed to the gills. If suddenly I could transmog anything into anything else, it would be even more difficult to choose what to keep and what to discard. Sometimes well-defined boundaries are actually kind of a relief. What do you all think?