The most concise praise I can give: it makes the vast majority of other FMs look like soulless pablum by comparison.
Much of the gameplay feels fresh and novel, despite never deviating from established Thief rules. It is always fair. There are no abstruse puzzles or micro-switches barring progression. So long as the player pays a reasonable amount of attention, there is nothing even unpleasantly challenging.
The environments evolve so elegantly, with every location refreshed once or even twice. What would be lazy and dull backtracking in most other missions becomes the soul of the level progression, and makes prior locations rewarding rather than repetitive. On top of that, it increases the realism: the world feels vibrant and alive, not just a static arrangement of assets.
And I haven't even mentioned its breadth, its minute attention to detail, or the creativity on show in every bit of terrain and object placement. There isn't a single cliché or tired meme to be found, which is miraculous after twenty plus years. It is simply one of the most astonishing FMs I've ever played. It's the kind of thing that makes me want to give up DromEd, because the bar has been set leagues beyond my reach.
My experience of it should have been diminished. I poked around some early builds, and had an initial real playthrough in a more frustrating beta version. Yet none of this took away from the grandeur of the mission, or its impact.
Tiny niggles could be had with it: there's a steep wedge outside the spot where you first enter Balnebrich's that's nearly impossible to traverse without jumping. There are additional places where OmniLights with negative value should have been added. The extinguishable candles stay frobbable after going out. But these are the tiniest of nitpicks, and do nothing to reduce its transcendent elements or warrant the removal of even a single point.
I also like this mission. It was a bit linear at first but then I liked the dynamic story and changing locations in it. The vague non-standart and not-so-connected storyline was also quite refreshing. The starting area was harder than other parts - with its forced ghosting - but I didn't find it impossible or that frustrating as some of the commentators here or on TTLG. I think that problem with that area lies in forcing player to play certain playstyle (ghoster) which might not suit his/her usual way of playing Thief. Still, for me it was just fine. It also reminds me of some old Ultima Underworld games - mostly in ITO's underworld)
You know, I think the supposed 'forced ghosting' at the start is one mistake that makes people frustrated. The mission doesn't end if you get spotted, so running around dodging the blackguards is actually a viable strategy -- it's also much more enjoyable than trying to stay hidden, and you make much faster progress in escaping. But players are so conditioned to hide in Thief that this doesn't occur to most people. I love it; it makes you reconsider how you should approach situations, and starts you off in immediate peril.