I love the idea, but there are a couple of problems.

The first is extremely large enemies. If you're fighting an Ancient Red Wyrm Dragon, what would happen after stealing its form? Would you become an enormous wall of scales, capable of smashing the confused red-tinged human? Or would you become a medium sized dragon and leave the dragon as a very large but flightless anthropomorph? Do you gain all of the abilities granted by the form, i.e. Flight and Crush and so forth? And these are all raised by one specific enemy. I'm sure if I could go through the catalog, there'd be many others, some worse re: the balance and rules.

In light of this, why not alter the ability to simply steal the enemy's type? If it's a dragon, you can become a dragon, gaining natural armor and wings but not stealing the specific form unless it's a member of the same type (i.e. small-to-large humanoid.) Keeps size a non-issue and preserves the flavor. for enemies' whose types are stolen, let them keep their overall form, but give them a simple debuff and have them treated as the Formthief's type for all effects (Charm Person anyone?) This prevents absurdities like having wings the size of sails going poof and reappearing on the 'thief's shoulders, just because he stabbed a dragon in the kneecap.

Another issue is the BAB. You're changing the core concept from a simple mojo-thief to a gish-thief. DR theft, attribute stealing and non-spell abilities each move you closer to the front line. Why not just push the BAB and HD higher and drop the Will save? Adding a couple of proficiencies might help as well, but they're not necessary.

Finally, have you considered SR stealing? I can't remember if it's a part of the spell thief base class or not. If it isn't, I think this class could benefit from it.