In a word, bad.
All mountains were volcanos that could erupt at any time, ...
Well, as somenone who lost several games of Civ I after popping my first goody hut - and "Whoops, you've encountered a horde of barbarians, your Civilization has been destroyed" I am used to stuff like that and consider it the genetic code of the franchise. Always kept you a bit on the edge and forced you to have some reserves and a backup plan - because things could and did go severely wrong when you least expected it. Actually the events in Civ IV were rather friendly and harmless compared to it's predecessors - and to make a difference and give you a challenge some of them could actually have been a bit more severe and have a bit more impact. Some of them could also have been implemeted a bit better, but in general I don't mind them - especially in single player mode. And as already has been mentioned: if you don't like them you can switch them off. Best solution imaginable to please everyone.