Since coming back from an exchange in Denmark (Aarhus) I've discovered that apparently, despite being one of the only native speakers among the other people there, I speak terrible English.
I come from the North of Ireland where our English accent has a very heavy Scottish influence and the difficulty people had in understanding me in Denmark, from both Danish people and others, was incredibly interesting.
So, what do you find hard to understand? I've always found English coming from a Spanish speaker generally a bit difficult, and people from Liverpool.