That is exactly why I don't like Donna. All of her episodes annoyed me. Martha was middle of the road for me, neither here nor there. Could have been better, I admit.
That episode was weird. But yeah, I guess I spoiled a bit for you. Didn't give any major details, though. Nothing you couldn't guess on your own anyways.
But Time Lords regenerating into the other gender isn't a new concept. It has been mentioned a few times, not only by Tennant, but I am sure Smith mentioned it a number of times. Not against the rules at all, just not something you've seen yet.