Those impressions have been overwhelming positive. That's only to be expected, since the Civilization series has been impressing reviewers for about a quarter of a century, but good reviews are never a sure thing when a series makes changes.
Civilization's core elements are still present. Players still guide a civilization as it develops from a single city in the stone age to a modern juggernaut. They still research technologies, develop cultures, and mercilessly grind their enemies into the dust. The difference is how players can accomplish the goals.
Cities have changed more than any other mechanic. Civilization VI is the first game to spread cities across multiple tiles on the map. Each tile holds a single district, which adds unique capabilities to the city. This adds a lot of strategy to city building, since players need to pick and choose specialties for their cities.
City states make their return from Civilization V and Beyond Earth. They still act as minor powers that offer bonuses to people who complete quests for them, but their diplomacy has improved. Each one provides a special bonus to civilizations that maximize their relations instead of simply providing resources, so allied city states can have a larger impact on a player's strategy than in previous games.
The role of religion has also expanded. Religions develop over time, and players can choose from a variety of benefits for their faith. It's even possible to win by spreading your nation's religion, so it can function as either a supplementary strategy or a primary goal.
Civilization VI has gotten rave reviews because it innovates and improves on existing mechanics without forgetting the core of the game. It's still a strategy classic that players can pick up and understand without too much trouble, but the changes make it a fresh experience for people who are familiar with the series.