The variety of different visual styles in the Gems of Provenance campaign is really impressive. In just the first mission you'll get to visit the mountain outpost, pagan garden, deep caverns and catacombs, and the Maw Of Chaos inspired magical realm of Mysteria -- and it's only just the beginning.
It is all complimented by competently written notes and an ambient NPC dialogue lifted from both classic Thief games that almost seamlessly weaves the events of this custom campaign into the official storyline. That such amount of work was done by a single author is quite an achievement.
The third mission -- City Public Water Works -- has another great, and in my opinion, generally underused, aspect of the campaign that deserves highlighting. You're tasked with stealing an ancient trinket located somewhere in a Precursor's ruins accessible through the water system of the Waterworks Museum. You start the mission as another museum visitor, so the NPCs are indifferent to your presence, and you get to explore the starting location without a hassle before entering the forbidden area and beginning your search for a ruins entrance.
Unfortunately, that's where the praise ends and criticism begins. The removal of the map is very unfortunate. Starting the mission by checking the map, learning the lay of the land, and choosing the plan of attack is an essential Thief experience. Even a compromise of having an incomplete map would work better. Like, for example, in an aforementioned Waterworks mission. The idea that Garret wouldn't have a map for a museum that is open to the public is ridiculous. But the access point to the Precursor's ruins could be something that you need to find yourself. The ruins themselves obviously wouldn't be on a map either, adding to the sense of mystery and discovery.
The removal of lockpicks is also a questionable choice. Having to wait on a guard patrol before picking a lock on a door, or hurry to open a chest and dive back into the shadows before the approaching footsteps get to you is one of the things that get your heart pumping in a game like Thief.
And, finally, there are the maps themselves. A few dead-end corridors aside, the first mission is completely linear. The second one rectifies at first by giving you a mansion to infiltrate and explore but ends with another linear stretch. But probably the worst is that sometimes the maps feel like they've been built without a real plan. As particularly evidenced in the Widow's Ire mission's mansion and a Keeper Castle of the final mission. Like there was an idea to build a mansion underground and have the player infiltrate it, but the order of the rooms themselves is probably got more to do with the author wanting to work on the main hall first, then the cellar, then the library, rather than how those would've been arranged in a real estate.