I knew about it and think it's typical knee-jerk, pre-election/vote grabbing legislation. Just like the original DDA, which was also installed by the Tories. 
None of my dogs have ever barked at the door/doorbell. Not sure why, as I've had a few large, guarding breeds - but they just haven't. Mind you, I always put them behind the kitchen dog-gate before I open the door anyway, more for fear of them getting out than them bothering anyone else.
Basically if someone is made to feel that your dog 'might' bite or attack them then they are considered dangerously out of control. Realistically, neither the police or the courts will want to be clogged up with people saying 'her dog ran to the front door, but didn't actually do anything else' or 'it barked but was nowhere near me' and it would be very hard for them to prove without any sign of injury anyway. I would say it's more likely to be if your dog jumped up and scratched someone they could, potentially, prosecute, whereas before it wouldn't have been possible. That said, I believe the DDA does refer to dogs 'causing apprehension' which is very vague and open to abuse and interpretation. I believe it does cover posties getting their fingers nipped through letter boxes though.
Truth is, you could have been prosecuted for any of those behaviours as soon as you stepped off your own property anyway, prior the 13th, yet it rarely happened. It was still only genuine cases of dogs growling, biting, knocking someone over etc that got reported and action taken. Not saying it has never happened, there will always be those that abuse the law to victimise people they don't like, but I don't think it's common.