Just wanted to say "thanks" to ANet for this feature. I'm the leader of one of the guilds in our alliance, and a regular member in another.
We have 6 guilds in the alliance, I estimate around 350 members total. We have had a few VERY isolated incidents where people are just getting used to the new access and saying "hi," and some mistells, but that happens anyway. Other than that, our guild members have been very disciplined in keeping the chatter to a minimum (or within guild chat!). I have not heard of anyone yet having to turn off AC due to others' spamming, nor has anyone had to be kicked due to AC abuse.
All it takes is a good code of conduct, and a good example by officers and leaders of its use. Thanks for giving us the chance ANet!