It's quite obvious when you think about it and as you (Victoria) seem to have a very in depth understanding of the mechanics of the game I think you'll agree.
1) Even though player and AI are on a level footing re resource dependant military units these tpetty matters don't trouble either the barbarians or the city states. Eliminating Barbarians early is even more critical as is earning envoys to gain control of key city states.
2) Due to no luxury resources amenity levels are very low and lead to low levels of hapiness.
3) Money is a constant serious issue for similar reasons. This is particularly annoying because you're desperate to upgrade the units you can to give you an edge but it's often unaffordble.
4) Traders become really valuable as a source of income.
5) Walls, walls, walls.
Im only part way through so there maybe more.