I love how this thread turned into "What Do You Not Like in Civ6"...
To answer OP (dullest mechanic): War.
To cite a classic: "War (in Civ6) is hell". The game is either quite easy if you do war or quite uncomfortable if you don't. Most players here advise for an early war against first neighbor, since it rids you of local competition and gets you bonus land and cities. Later war isn't that easy to lead unless you overpower your enemy... But if you get on a conquest spree with the army you built in ancient era and don't lose units, you will trample anyone. You don't need to build more units, which makes building an encampment very one-trick (for getting a general). The AI has little tactical abilities and is unable to focus on a target or any kind of goal. (last time Khmer conquered my unwalled city, provoking an emergency, resulting in me getting free 6k gold since it withdrew half the army from the city).
1 unit per tile was discussed enough. It is not a good solution for a dozen units involved in conquest. The map, while playing the map is a good idea, is too small for tactical combat. It's as if Napoleons armies were spread across half middle Europe when fighting the battle of Austerlitz, with one artillery at Vienna and the other at Prague both aiming at the same fort. Also, moving more than a dozen units is extremely tedious to do every turn for 150 turns. Half the time it will only be finding a suitable spot to move on the given turn, not doing anything worth mentioning.
City spread is also crucial. From medieval up there is little area not covered by city attacks - if the cities are very clustered, half your units are being fired upon each turn, some even from multiple directions. It would be okay by time of artillery - modern era, but not quite in medieval warfare.
I played little multiplayer to evaluate war of that type. Players tend to delay conquest to a suitable time, when they have advantages or have finished their infrastructure. The simultaneous turns make Civ6 into half-turn-based, half-RTS. In case of war, who clicks faster is victorious... A rather unexpected result in a traditionally turn-based game, as it differs significantly from single-player experience.
I remember AI personality descriptions from Civ2: there were expansive militarists, perfectionists, aggressive ones, etc.
The current gameplay can be described in these terms. Perfectionism (city building) is the basis upon which stands military expansion (conquest), which is merely optional. The complexity of city management is a must, war is optional.
Too often defense is not even required (if only to provide a trap). Offensive action is discouraged, both by game systems (diplomacy, war weariness) and additional complexity in frustrating conditions. Standing army is a production and gold sink.
tldr: War is dull.
