I'm flying in Florida right now out of a Class D airport (Space Coast).
It's pretty informal. Visiting pilots often ask the tower (on tower freq) about where to eat, where to stay etc.. I don't think that would go down too well in the UK...
Some other differences - they have this thing called "Cleared for the option" which means you can do a full stop landing or a touch and go at your discression. They also do ILS backcourses and circling to land approaches which I don't think the UK permits either.
Also, when you are doing circuits (or patterns as they call them here), when requesting departure you call "xxxx is on the go."
Altimeter settings are in inches of mercury, not millibars and they never use QFE. QNH is just called "Altimeter setting".
They probably come over as more friendly, less intimidating than UK ATC but it is sometimes frustrating when people are 'chatting' to the tower and you are trying to get a clearance.