I think it's less the ease of annexing minors and more the massive power boost you get from annexing your first city when you're playing a 1-city start. Since minors have about the same starting population as a major, annexing one of them basically doubles your population and starts the snowball ...

If I recall correctly, he did some basic math before writing the first book and thought it was stable, but then after it came out a bunch of astrophysicists started writing him telling him that the thing wasn't stable and would slide into the sun... so in the second book he added a bunch of engines ...

Aside from the model design interface which most folks seem to want to see improved, the main thing that annoys me is having to manually predict resource consumption/production for construction. To avoid construction delays I have to go through each zone, tally up the difference between the zone ...

Had this thought when playing; a ringworld type "planet" could be a very cool map. No mining or oil, but lots of water and a reasonable climate, or not if the ring is starting to break down from age, neglect, or the dissolution war. Could make for a rather unique campaign playstyle.