Could you give an example of useless complexity in non-Civ4?
I have thought about it and have a hard time giving a precise example, sorry. What I had in mind, specifically, is the way Europa Universalis 4 has Menus (with Options) within Menus (with Options) within Menus (with Options) within Menus (with Options), etc. seemingly indefinitely, whereas very few of those options seem to matter.
On your other points, you're quite right that some mechanics in Civ4 are quite obscure (Granary/Overflow). They require learning and "arithmetics in the mind", as you put it.
Some other mechanics are simply not elegant and rather poorly implemented (Diplo). Diplo has mostly hidden factors but those have been researched through the years and are now well documented. You may need to do some research, though, if you seek some information on, say, peace vassaling.
On the management front, which is the main front to me, I appreciate the way I can easily access the information I need. Tile yields and city yields, namely.
I don't agree. 1

=3

on green hill mine. But others told me I wasted 5 worker's T when the worker had to chop... ))
This is not what I meant. 1F = 2H = 3C is an old conversion ratio, used to help prioritize / weight tiles.
It means you can sacrifice 1F in order to gain 3C, for example, and you should be roughly the same. If you sacrifice 1F to gain 5C, you're making a good deal. If you sac 10H to gain 10C, you might have something better to do. If you have the choice between working 3F vs 2F1H, then 3F is likely better.
It's not a hard rule, mind you, and it's also contextual (sometimes 10C will be better than 10H).
On your example, the green mine is 1F+3H, not 1F=3H. The tile you should be comparing it is a forested unimproved tile, so :
1F3H vs 2F1H or 1F2H or 3H.
If you're building settlers/workers, you're comparing a 4 yield tile to 3 yield tiles. Unless you're CRE/IMP, the mine only gives you +1 yield per turn. Compare that to improving pigs (+3) and you understand why
others advised you to chop (+20). The chop is a one time thing but settlers/workers are very special items, since they
produce.
You want them out asap, so they start producing asap. Therefore, we are not comparing working the mine for 20 turns vs chopping the forest. We are comparing getting the settler on Turn X to getting the settler out on Turn Y (and chopping wins).
Note that sometimes you have the time to build your mine and then chop the forest and still get the settler out on the desired Turn X.