Well, this is either a bug or the most PVP game-breaking thing since sliced hunter -- but apparently Polymorph no longer heals targets on the test realm. The various forum threads include some pretty hilarious mage justifications of why this should actually be a feature, most of which involve tirades against warlocks.
The main problem with Polymorph not healing is that it's a long CC that's somewhat difficult to break. Without the healing, mages could just poly, frostbolt, poly, frostbolt, ad infinitum. Mages are quick to point out that locks can seduce/fear and DOT in a similar way, though, although sheep isn't subject to the same diminishing returns.
There's a good chance this may be a side result of a change in mob Polymorphs, which have been shortened to 8 seconds from 20. But just in case this makes it to live, I think I'm going to be binding Cloak of Shadows and Vanish to about half of my keyboard.
What do you think? Is it a bug or a buff? Should sheep heal at all?