I'm an American (born and raised in South Texas) with a neutral American accent, also an ESL "teacher" living abroad, so being able to understand broken English is vital to the job. I've been living and working in WuHan, China since 2006, and the Chinese are notorious for making what I consider simple errors, even when they've gotten to a level that I would consider conversational.
They're always adding +ing, they can't grasp past tense, they always forget the use of 'a' and 'the', and they often mix up 'he' and 'she' even in conversation.
These problems in their English stem from their own language, as Chinese at it's root is actually a pretty "simple" language complicated by it's use of tones and a non-Roman alphabet (although they have PinYing but that's not fully accepted amongst the older generations). Chinese does not have a difference in 'sex' so you always say "Wo" me 'I' or 'Me' adding -da for 'mine', "Ni" for 'you' adding -da for 'yours', and "Ta" for He/She adding -da for 'His/Hers'.
However, even though they make these 'simple' errors, they're able to ruthlessly dissect sentences into their parts of speech, something I forgot about primarily after High School.
I've thought about this very topic, and it occurs to me that English is a very 'descriptive' language and to people whose mother tongue isn't as descriptive having words that mean the same thing with varying degrees of feeling might throw them off. Also, at least in America, English is taught all through school from Elementary (Primary) level, to University. The only difference, at least in Texas, is that in the Public school level it's being taught in a way to help pass a test (
TAAS which has been replaced with the
TAKS evidently) because the school received funding based on the number of people who passed the exam. To complicate the problem, most Americans, or Ostridges (why is it saying this word is misspelled???), never leave the USA except to maybe Mexico or Canada where English is understood. So, they see no reason to put a lot of effort into their studies after school. Mixed with SMS/IM/IRC/Chat and you've got a deterioration of the English language. Finally, the celebrities (mainly HipHop stars I think) often use their gangsta talk and butcher words to make them sound 'street'/'urban' or fit a song which I think is a form of pollution since the youth latches on to these words and it becomes widely used. Just look at
Urban Dictionary for examples.
However, as more and more Americans are starting to leave the States as either EX-Pats, or world-wide travel, I'm hoping that this will change. I work with an older woman from England who is constantly saying that the level of spoken English in England is atrocious and appalling now. I will say that my biggest problems has always been spelling. I've always struggled with spelling, even though I was one of those kids that read the dictionary in my free time. To this day, I still struggle with it.
Long post, guess I had a lot to say.
*EDIT* oops forgot:
I would also agree that accents have a big part in understanding. I've worked with people from all over the world: Canadians, Americans, Northern and Southern British, Cameroonian, Rwandan, Irish, Australian, Chinese, Indian, Pakistani, Saudi Arabian, Macedonia - and I have to say that sometimes I have a hard time understanding words the Irishman says, and we had a teacher with a ****ney accent from England that I had problems with. The guy from Cameroon spoke English with a French accent, and was sometimes difficult to understand. I also find that if students aren't understanding, they just shut down.