Iran isn't really a problem (legendary hospitality ), but Afghanistan (probably the most difficult at the moment with a war currently ongoing, not you have to keep a eye for potential kidnappers but you have to watch the sky as well) and North Korea may well be difficult (though not impossible as long as you are prepared to take along a couple of Government issue Guides/interpretors );

http://www.highplaces.co.uk/treks/country29.html

I personally felt safer on a trip to Cuba (even with the Hurricane) than I did with a trip to Barbados for example, although it has not been unknown for me to go drinking in the local pubs and bars in an area described by the UN as one of the most dangerous locations in the western world.