I've been to 129 countries so far including some of the harder ones listed, Belarus, Bhutan, Syria, North Korea and Afghanistan. Mostly what I have left is Pacific islands, Central Asia (though will be there in May), Central Africa and Eastern Europe.

Currently I'm waiting on an Iranian visa authorization, apparently it's taking ~90 days to get approval for Americans right now (and Americans have to be on a guided tour). Saudi will also be difficult to visit on anything other than a transit visa. Turkmenistan is difficult, either need an Iranian+Uzbek visa to get a transit visa, or take a guided tour. Angola is almost impossible (or very expensive) to get a tourist visa, and even transit visas are not being granted from what I hear on the lonely planet site. I've heard Algeria can be a pain to get a visa as well. Libya used to be impossible/difficult for Americans.. not sure now what the situation is.

Nigeria isn't so bad.. getting the visa was relatively easy actually. I just did a Nigeria/Cameroon/Gabon/Equatorial Guinea trip last month. I skipped out of Lagos pretty quickly though.