Civilization 6 makes multiple dramatic changes from its predecessors. It will take more time and games before it all shakes out but here are some areas I found interesting so far.
Linear yield instead of multipliers
In Civilization 4 a Library would give you +25% Science and be followed by a University that would provide another +25% Science. Civilization 5 mixed in constants with its multipliers (ei: +2 Science and +25% Science). In Civilization 6 there are no building level multipliers. It is a flat +2 Science from Libraries, +4 Science from Universities and +5 Science from Research Labs.
In the past multipliers served to accent the differences between cities. A city on grassland would have more food but less production than a city on plains. After both cities got a few food and production multipliers the gap between the cities would be even larger.
Constant bonuses served to minimize differences between cities. After each city has built a few +2 Food and +2 Production buildings the differences inherent to their terrain becomes watered down by their buildings.
Civilization 6 avoids the homogenizing effect of constant bonuses because of the district system. The adjacency bonuses from districts can provide as much yield as the first or second building. Because cities can only afford two districts in the early game it is critical to pick the specific districts the city can make the best use of. Cities will end up being different because they house different districts, even if all science districts play out the same.
Linear return on investment
Because almost every building provides a constant amount of yields the choice of what building to make is pretty simple. Count up the yield and divide by the production cost. The yield provided by a Library won’t grow down the line when the University is built. Each building can be evaluated in isolation. Districts are the saving grace, as planning ahead for adjacency bonuses and housing thresholds is important and interrelated.
Unit movement and builders
Units are no longer able to execute a move if they don’t have all the movement points needed to take the step. What this means is that a unit with 2 movement points can not cross a flat tile (using 1 movement) and then go up a hill (requires 2 movement). This change has a huge impact on how quickly units can traverse the map and how easily melee units can close the gap with ranged units.
I am still getting a feel for the implication of the change on combat and army composition but it has a frustrating effect on managing builders. Often you’ll plan out what actions you want to take with a builder only to have it require multiple turns to execute due to this movement change. This was a problem in the bad old Civilization 3 days when workers could only move a single tile and it was the main reason workers in Civilization 4 moved two tiles.
AI Combat Strength bonuses on higher difficulties
In Civilization 6 the AI’s units get a strength bonus based on the difficulty level. This is different from previous Civilization games where the AI would get bonuses to producing units. Before you would be outnumbered but tactically equal units.
Success in combat involved controlling the size of the front such that you had a local advantage even when outnumbered. With the tactical bonuses it may now be tricky to be able to fit enough combat power in a small enough area to kill the AI units.