It really depends on location, just like British English does. As James said, there are many people in the UK who speak badly, as well. I can't say I've ever heard of someone greeting someone else in business by just saying, "Hi." This seems much too short, and would be awkward. They may say, "hi there," which I don't see as being too familiar, just friendly. The language spoken in cities in the Midwest of the USA is considered the acrolect of American English, which means it's the most proper, and accepted, form. The difference between proper Midwestern American English and proper British English just comes down to pronunciation, some differences in vocabulary, and many differences in spelling. Many Americans do think that British English sounds more "refined" though, but they also frequently think it sounds over-the-top. For instance, there are a lot of American women who will attempt to use a stereotypical British English accent when they say something silly. British English is often made fun of over here, although I'm sure American English is made fun of quite a bit in the UK, as well. Really, all it comes down to is location. The further into rural areas you go, the more simple and unrefined the language will be. The same can be said of mainly blue-collar, or working class areas.

American English, at the higher end (academic), can sound and read as good as British English. Unfortunately, too many Americans speak at what I would call the "lower end". Their English, to my British ears, sounds over-familiar, simplistic and unrefined. You do, however, also get plenty of people in the UK who speak very badly and who have very little idea about how to talk correctly.

My advice to you is: try to speak British English, because the American English is too simplified.

In america, I say "Yo joe!" :P