Some of my thoughts on the missions I've played thus far:
Order of the Dew - a very short, but well-made mission. Despite being small, it rewards exploration, and has multiple routes to your goal. Just wish it lasted longer than 15 minutes.
Lord G - feels very unfinished, has some game-breaking bugs. Nice-looking mansion exterior, but it needs about a year more development time, and a hefty dose of testing.
Downtowne Funk - would be easily identifiable as a Yandros mission even if it had been submitted anonymously. A mostly well-made small city mission that tries, to a certain degree of success, to feel like a natural extension of one of the original missions. The reliance on some original LGS areas means that some areas feel better quality than others, and it's also one of the easiest of the missions I've played. Nonetheless, good fun with an interesting story and a lot of hidden loot.
Making a Profit - this one gets my award for the mission I most wanted to like, but couldn't quite get on with. It's enormous, to the point that the scale feels off, and to compound that, it's full of gameplay that feels like a slog - many long lockpicks, an objective where you have to lug a body around, and you are sometimes left directionless, which is painful in a mission of this size. At it's core, there's an interesting, well-written (if predictable) story, with some good voice-acted cutscenes and fun progression of objectives, and it has some nifty puzzles. But it badly needs the fat trimming.
Darkness Walk with Us! - it's a Haplo mission so I knew what to expect going in - a tightly designed, fun, well-paced puzzler, and this mission didn't disappoint. The architecture is workmanlike, but the structure and pacing, the way the mission gradually expands its scope, and the puzzles which are balanced to usually make you feel clever without becoming stuck for long, makes this one probably the most pure fun I've had with any of these contest missions.
The Madness of Wolfgang Handspiegel - definitely feels like a small/beginner mission. It's very small, and relies on a linear sequence of find-the-key, find-the-hidden-switch mechanics. The few rooms that are there are nicely made, but there needs to be more of it, and less linear.
Heart and Soul - fun and atmospheric, the main issue I had with this one is that there's not much pacing or guidance given to the player, so you find yourself running around a large, mazelike environment, hoping you stumble on what to do next. It kinda reminded me of Hexen in that way. Some fun new undead creatures and lots of secret passages make it good fun to explore, even if you are completely lost!
Lost Among the Forsaken - probably the heaviest use of custom resources I've seen so far in the contest, but they don't go to waste. This one is beautiful, and the way the environment is constructed in 3D space is mindbogglingly complex. That is really my only criticism - it's easy to get lost due to the sheer complexity of it. The atmosphere is top-notch, the story elements are integrated well and serve to bolster the atmosphere without distracting from it, and all in all, it's probably the best "old quarter" style mission I've played.
An Enigmatic Treasure with a Recondite Discovery - some of the environments look really nice, but the mission seems built to too large a scale, there's little reward for exploration and there's a lot of pointless dead ends. The writing is prosaic at best, and could do with a lot of proofreading. The biggest problem with this one is the unforgiving difficulty - swathes of marble floors, no consideration of shadow placement, souped-up enemies. It's hard without being a challenge, it just feels unfair. The only one I've actually given up on so far.
Dewinder Manor - as others have said, this mission is basically a beginner-level mansion heist, doubling as a complex forum injoke. For what it is, it's not at all bad, and a fun way to kill half an hour.
In & Out - a mission that starts simple, and expands into multiple underground layers that serve as a Greatest Hits tour of Thief 1 environments. The story is essentially just a barebones excuse for this downward jaunt, but the ride is well worth it, as the gameplay shines. The readme said the mission should be ghostable, so I gave it a try, and it was some of the most fun I've had trying to ghost a Thief mission. Highly recommended.
Mother Redcap's Last Request - another basic/beginner mission with simplistic, empty architecture and a play-time of under half an hour. But, as with Dewinder Manor, it doesn't do anything wrong design-wise, it's just simple. The story is well-told and relatively original, and it's good fun while it lasts.
Rose Garden - might catch some flak for this, but I really didn't get on with this mission at all. It has atmosphere in spades, and a huge amount of places to explore. But it's overwhelming. It is surprisingly linear in some ways - despite being an enormous city mission, it's actually extremely difficult to navigate effectively, with often one, labyrinthine route to get to an area. There are scores of doors, ventshafts and the like that don't lead anywhere, some of them only accessible by tricky climbing, which makes exploration feel less rewarding than it should. There are tons of extremely well-written texts, but most of them go into your inventory, meaning I have no idea if they're vital to progress or not, so by the end, it was taking me a good minute to scroll through my inventory. Maybe I'm just not clever enough for this mission, but I constantly felt like I was missing things because there were obscure clues that I didn't understand. The enormous size of the level, coupled with the often very difficult climbs needed to just get around meant that I spent most of my time with this mission feeling lost and frustrated. The plot was well-written, but by the time I reached the end, I'd basically forgotten what I was doing and why. I have no doubt many others will love this mission, but I barely had the brain-power or patience required to finish it.