AT&T is going call their HSPA+ (normally considered fast 3G), 4G, like T-Mobile. They're also going to start deploying their LTE network (Verizon's type of 4G) starting this summer. For an explanation between the two.
Unlike T-Mobile, who only have HSPA+, or Verizon, who only are going to call LTE "4G", AT&T is going to call two different technologies "4G". I could see that getting very confusing, explaining to people that they can't get this type of 4G here, but this phone supports that type of 4G.
I would expect all Carriers to go to some sort of tier pricing, figure they will at some point make you pay more for watching videos on YouTube as well.
To me all the extra charges are not worth it, however to some they will be.