The vowels drive me nuts!, in Spanish my native language, an A is always pronounced in the same way (as an A of course), in english you have several ways to pronounce the same #$!#!~& letter!, for example all these words have different pronunciation:
Fat, far, up, until, mouth
I can barely notice the difference, it is not difficult when you are listening but it is really annoying when you are speaking and the other people do not understand you because if this subtle (for me) differences, my last experience was in a store I asked for a Bag and the clerck looked at me attonished (of course she understood Bug, could anybody told me the difference!!!)
Of course I am planning to take a pronuntiation course, and I am having fun really, I havent had any real problem because of it (I am taking it easy
)