Ric, only people that know enough British people would understand the regional dialects/accents. There aren't that many either, by my recollection. The only difference is that British accents aren't often clearly outlined and portrayed in cinema like you would see in American cinema (with "angry New Yorkers" or "dumb hicks").

No one is saying learning from the movies is bad, just the way in which your statements make it sound like people from Los Angeles (and thus, Hollywood) are only perceived as American English, when it is actually the proper way to speak. Nevermind the fact that a wide array of accents have been in American movies, especially with many taking place in New York. I don't believe anyone thinks you 'insulted' anyone, just that you are wrong. Please don't take offense, but as someone who lives here and has family coast to coast, you are wrong.

Haha, you bet Su.Ki.! They actually subtitle television shows here if someone has a thick enough accent! I know I can't understand half the things southerners say.

While many American actors live and work in Los Angeles, they didn't necessarily grow up there and instead have their own regional accent which, depending on how strong it was, they may or may not have modified to a more standard sounding accent. 

Hello Alex. Yes, I think that you are absolutely right.

There is comparatively little difference between <em>standard</em> American English and <em>standard</em> British English. For example, a Harvard professor and a British academic from Cambridge would speak to each other in virtually identical language.  A tiny proportion of their vocabulary and structure might be fractionally different, and although there would be some differences in accent (vowel quality, rhoticity, the odd alveolar flap), these would be relatively minor.

What is far more significant than any difference between English-speaking countries are the differences <em>within </em>countries. I don't think it would be an exaggeration to say that the Cambridge academic would find it easier to understand the English of his American counterpart than that of a Scottish fisherman or Liverpool schoolboy. Likewise, the Harvard professor would probably struggle to understand a Texas farmer or a Harlem teenager more than he would his fellow academic from England.

As you say, the core of English pronunciation - and also grammar, vocabulary and all other languages features - really is pretty similar.

Ric, mostly the implication that the <em>perceived</em> American accent was due to Hollywood, when it's not. It can be properly defined, and the outliers of proper American English pronunciation are those in regions like the New England states or the 'South'. Even then, there are plenty of people in those regions who don't have a <em>regional</em> accent. For instance, there is a pretty big difference between downtown Boston and the Cape Cod region of Massachusetts.

On to the topic at hand, I think in truth, there isn't that big of a difference between many of today's dialects. However, in English-speaking countries, there can be sizable differences depending on the region, much like the differences between New York, the Bible Belt, and the nearly-Canadian northernmost states.

In fact, Su.Ki., I'm not sure what part of the country you are from, but would you say there is <em>that</em> big of a difference between an American accent and a Londoner? I'm aware of other regions with some nearly unintelligible slang, but to me it seems like the core of English pronunciation is still pretty similar in most ways.

Ric, I think people voted your comment down because it's simply untrue. The rest of the world <em>doesn't</em> think that there's only one American accent.

We are all aware of the differences between the accents in various parts of the USA. Even people who've never set foot in the country have seen countless movies and TV programmes set in New York or Texas, for example, and can easily tell the difference between a NY twang and a southern drawl. Most of us can also spot a New England and a Midwest accent too.