Brattiness sure can be a fine line and many shades of grey. Here is the way I think of it:
There is the playful brat that may get you a smart smack on the butt or a task that is more fun than discipline. An example that comes to mind is if Master were to say to me "I am going to use the bathroom and I reply "I didn't hear you ask permission." It is certainly bratty, but Master knows it was in fun and has a hard time hiding his smile.
The next level may be to argue a point or to take too long to do something that was asked of me. This is not good submissive behavior, but it is usually not done with the intention of being disrespectful or disobedient. Just that sometimes that old vanilla girl escapes for a few minutes until Master gives a hard yank on the figurative (or maybe even literal) leash to remind her her of her place. This usually earns me a writing task of some sort.
The third level I would not refer to as brattiness at all. That is when a sub is willfully disobedient, dishonest or disloyal. This behavior can earn a submissive her walking papers, or if she is lucky a very strict punishment.
I have actually been guilty of all three levels, and let me tell you I have only done level three twice and both times were in the very first few months of my training period. I tried to test the waters and nearly drowned. I am very lucky to still be with my MstrDragon. "Peace" ~blizz~