As an English teacher, I think one of the most difficult things for students to master in the English language is the pronounciation. I ran across a clip from The Ellen Show where she talks about one of the many websites that offers help to foreigners learning the language. A lot of websites will allow you to type in a word and then hear the correct pronounciation. However, she highlights a website that is set up to look just the same but actually is telling people wrong pronunciations. It's mean I know, but please, just watch, it's quite funny at the same time.
There are some very common mistakes that I see here in people pronouncing incorrectly. The first main hurdle is the TH sound. It doesn't exist in Spanish, so as a result it is quite difficult for them to master. So, for example, if you ask a Spanish person another word for Dad, and they want to say Father, it will more come out like Fader.
Another quirky Spanish thing is their inability to pronounce a word starting with S. I would say it is almost virtually impossible for a Spanish speaker to say Spain, because they in thier language they always stick an 'e' before that consonant. So, Spain becomes Espain. Special becomes Especial (which is the actual word for special in Spanish).
With our plethora of silent vowels and consonants, it's easily understandable why some words just leave people speechless. Take some of the following words that are not pronounced how they are spelled whatsoever: clothes (we practially don't even say the TH) or February (although I can't really say it without the first R myself), and don't even mention American cities or States - Illinois, Phoenix, Arkansas...impossible for any learner!
On a similar note, there are a lot of words that ARE pronounced the same but screw learners up with thier spellings. Honestly, how can 'through' and 'threw' be pronounced alike? 'Bye' and 'buy', 'weight' and 'way, 'hear' and 'here' or 'write' and 'right' - the list goes on and on.
Tricky and hard to ever pronounce like a local are words that to us completely different but to a Spanish are homophones (they sound alike). Some examples are 'feel' & 'fill', 'bitch' & 'beach', 'desert' & 'dessert', and my favorite 'beach' & 'bitch' which could get you in a lot of trouble!
When you start talking about differences in accents between us and other English speaking countries, the difficulties for learners multiplies, but thankfully there are websites out there that help them learn - obviously not the one Ellen found though!