A few friends of mine in Facebook added an app called "Cities I've visited", and after filling it myself I found out and confirmed a couple of interesting things.
Even though I've been pretty much to all the countries in what is traditionally considered Western Europe, I'm still missing Switzerland, Greece and Ireland. Furthermore, even though I've been a few times to France, quite big parts of the country are still virgin territory to me. I already knew that I'm missing southern Spain & southern Italy, as well as huge swathes of Eastern Europe, and funnily enough after all these years I haven't been to Norway or Denmark yet (many times through Copenhagen airport, but not to the city itself).
I have been to the big 3 countries in Latin America, but the rest are still virgin territory. Same with most of Asia, and all of Africa and Australasia. Definitely still a lot to be discovered