I agree, it depens on what you want to do! Someone wanting a pleasure horse might just want it to be bombproof.
As I ride dressage, I would consider the 6 classical steps (rhythm, suppleness, contact, straightness, impulsion, collection), but every horse and rider will always be learning. A horse cannot be finished if they are missing one of these steps, so I would consider it "finished" when the horse can, without a struggle, show every one of these steps at it's best capability. (We cannot expect a QH to have the same movements as a WB, but the QH masters all 6 steps and uses itself, it will make **** good competition.)